Making a 1:6 scale ‘working’ fireplace

The cosy library set featured in the previous post included a fully ‘working’ fireplace. Any ‘flames’ needed were to be added in post-production using CGI so I didn’t have to worry about those. But my brief was to make the physical prop work .. that is ‘light up’ .. to an extent, at least incorporating a suggestion of glowing embers. At the time of making it was not yet clear whether the ‘coals and logs’ part would be seen in different states i.e. from fully stocked to nearly spent, so my thinking was that this part needed to be made as a separate and interchangeable shell .. a translucent one .. independent from the source of light. In any case, I didn’t want to mess with integrated electrics since they’re almost always a bit cursed, in my experience. So I decided to make the fireplace setup open at the back so that it could be lit from behind as simply as possible. Below is the only photo I managed at the time of the fire lit up, a quick test in daylight before the surfaces were fully painted and accessorized ..

David Neat, props for stop-motion animation, working fireplace effect, painting unfinished

I designed the ‘coals and logs’ unit to sit within an ornamented grate which hid its edges and also masked spillage from the light source behind. This meant that it would be easy and quick to substitute different stages of the fire modelled on the same base-shape.

David Neat, props for stop-motion animation, Sculpey modelling of fireplace

David Neat, props for stop-motion animation, Sculpey modelling of fireplace

I chose to make the base shape in Kapa-line foam, probably because this was easiest .. but Super Sculpey doesn’t readily stick to much, especially foam, and to have any control over the modelling a firm base layer is essential. So I started by massaging small portions of Sculpey to become almost paste-like and working them into the surface. Once this was covered the resistant, wax-like qualities of Sculpey could be fully exploited .. I much prefer to model by pushing/impressing, kneading and displacing, hardly ever cutting or scooping out, and a whole variety of weird impression tools will often do much of the job for me. I had a bag of strange, impossibly hard and oversized ‘croutons’ I’d found in a Chinese supermarket and I didn’t have to do much with these to create an interesting textural starting point. When it came to the ‘logs’ or chunks of coal I used a custom impression tool I’d made for tree-bark .. Sculpey modelled and baked over an old scalpel handle .. using it in a partly random way, just to create some spontaneous interest.

David Neat, props for stop-motion animation, Sculpey modelling of fireplace

David Neat, props for stop-motion animation, Sculpey modelling of fireplace

But the whole looked dull, deliberate and lifeless, until I attacked the surface with brushes! The most successful was the black plastic one, like a large and sinister toothbrush, which accompanies wire brushes usually in packs of three .. I’d never found a good use for these plastic ones before, and none other since really! Once the Sculpey work was finished I made a standard mould from it comprising a silicone ‘skin’ part supported by a plaster jacket. This is common practice, even though it takes a little longer to complete than just pouring a block of silicone, because it cuts the amount of silicone rubber used to easily under a quarter.

David Neat, props for stop-motion animation, Sculpey model and silicone mould

David Neat, props for stop-motion animation, casting clear resin fireplace

I had two options for casting the hollow, translucent shell .. using either clear epoxy or polyester resin. But epoxy resin can only be made thixotropic (converted from a liquid to a spreadable gel or paste) by adding a filler powder such as fumed silica. Dependant on the amount of powder needed, the epoxy resin could lose much of its transparency, whereas clear polyester resin can be thickened using a specially thickened  gelcoat  additive which is almost as clear. I needed to mix the clear polyester, gelcoat and shared catalyst together first before tipping the mixture into the mould. I had to wait about 15mins before the mix firmed up enough to be ‘shaped’ into a relatively even shell, using a chopstick as a spatula, but the window closed fairly quickly after that.

David Neat, props for stop-motion animation, casting clear resin fireplace

I’ve said ‘clear polyester’ but in actual fact it was the ‘general purpose’ or GP polyester resin from Tiranti, not the ‘Clear Casting’. I’ve always used Tiranti polyesters (whether general purpose, ‘multi purpose’, ‘clear casting’, gelcoat or thixotropic paste) partly because I’ve never had any major problems with any of them. They’ve also lasted far longer than any others I’ve bought .. for example, I used the same can of GP polyester on-and-off for over five years! Tiranti’s GP cures a warm grey/beige which can be seen from the following photos, but this was fine for my purpose, and the cloudiness (compared to Clear Casting) was also something which I’d hoped would diffuse the light for a better effect.

David Neat, props for stop-motion animation, resin and Palight fireplace unit unpainted

David Neat, props for stop-motion animation, resin and Palight fireplace unit unpainted

David Neat, props for stop-motion animation, resin and Palight fireplace parts unpainted

I removed the cured polyester shell from the mould the next day, and designed/made the ‘stool grate’ (that’s the proper term) around it using Palight foamed PVC. The photo below shows this primed in Humbrol matt black enamel (not yet given its metallic gilding), set up against the fireback and the hole cut to let the light through. There were restrictions to the depth that the fireplace unit could be, and I could have solved this with much more blackening or shading around the stool grate .. a shame, but there was no time left. What did work nicely were the strips of vinyl wallpaper I used to suggest the fireback stonework, washed and sponged with acrylic. 

David Neat, props for stop-motion animation, painted fireplace parts

David Neat, prop and set making for stop-motion animation, working fireplace effect

To give the rich ember colour when lit, I had thought of coating the underside of the polyester shell with red/orange/yellow glass paints (i.e. Marabu GlasART or Pebeo Vitrail) which I know work very well. But it the end I felt it would be more adaptable if the colour came from the light source, or through gels fixed behind the cut hole. Since I’d spent some time on the modelling (especially on getting the texture interesting) the painting was fairly simple .. an overall skim in black first with a large ragged brush, followed by less of a skim in mid-grey and then even less in light grey. Again I used Humbrol enamel for this, just to be sure the paintwork stayed on the polyester surface if the piece was going to be handled.

David Neat, props and set making for stop-motion animation, fire effect

I used a thicker Palight for the fire surround and mantel shelf below, and the small ‘designs’ were cut/carved using the thinnest .. 1mm. Palight of whichever thickness can be carved and sanded with surprising ease .. it’s a lot like carving a soft wood, but without having to cope with grain direction, and the exposed ‘grain’ hardly looks any different to the rest. Here in the UK it’s available in white up to 10mm thickness from Bay Plastics at http://www.plasticstockist.co.uk  though from 2mm onwards it’s actually ‘Palfoam’, which is an even softer variant.

David Neat, props for stop-motion animation, fireplace carving in Palight foamed PVC

David Neat, props for stop-motion animation, painted fireplace surround

For creating a controllable ‘speckle’ with a slight sheen to it I base-coated first in a lighter tone then mixed darker acrylic with some acrylic retarder gel, to stipple it over. This allows a decent amount of working time in which to even out the effect and it makes the paint into more of a glaze. Most of the tube acrylic paint companies offer their own brand, though one will work with another, however the ‘gel’ type has become less common. Now it’s usually a thickish, glycerine-like liquid but it should work in the same way.

David Neat, props for stop-motion animation, fireplace setup nearing completion

Thanks again to Astrid Baerndal for the only photo I have of the fireplace installation properly assembled, under natural light with no atmosphere unfortunately, in the hurry to ship the whole model off. The large fish were modelled in Super Sculpey over Styrofoam base-shapes; hollow-cast in polyurethane resin; basecoated in Humbrol matt black enamel, then ‘dusted’ with Treasure Silver Wax Gilt finish like much of the rest. More about the making of the fish can be found about a third of the way down my general article Modelling and shaping, one of the group Making realistic models which is first on the menu in the Methods section.

Casting prop books and making ‘specials’

David Neat, props for stop-motion animation, cast and painted books c 1:6 scale

Continuing with the subject of prop-making for stop-motion animation, back in 2011 I had to make a small library full of books for one particular film. I made both the sets and props, including furniture, and the heads of the puppets for this one. The setting was broadly based on Horace Walpole’s Strawberry Hill so the books had to look ‘antique’ but with a little more freedom in the choice of colours. Most of the books on the shelves needed only simple surface treatment, and could be faked because they weren’t going to be taken out or touched, so for the most part it was sufficient to create ‘blocks’ of convincing frontage with some suggestions of depth at the sides and tops. But there also needed to be many piles of loose books on the floor and on tables, plus a proportion of loose books in the shelves, and a few of these actually needed to be opened! Below is a close-up of part of the shelf-book frontage with singles interspersed. Many thanks to Astrid Baerndal http://www.baerndal.eu for this and countless other excellent photos in the past!

David Neat, props for stop-motion animation, cast and painted books in shelves, c 1:6 scale

Since all of the books .. whether faked blocks, simple or more involved singles .. were made in polyurethane resin, the painting method was basically the same. The castings have to be left for a few days to fully cure; then they need to be lightly scrubbed in warmish water and detergent; then primed using a plastic primer such as Simoniz or Rust-Oleum; after which they can be painted with regular acrylic using whatever preferred methods. I used a mixture of my usual acrylics .. DecoArt ‘Crafter’s’ or ‘Americana’ also Rosco Supersaturated and in addition Vallejo Model Color for fine details and transparent glazing. Given the prominent ribbing and other textures the ‘worn’ look was easily achieved with a combination of careful sanding with a sponge-backed sanding pad and some dry-brushing. The film-makers agreed that any attempt even to suggest writing on the books would have been too overwhelming in effect .. quite apart from the effort, since there were many hundreds of them!

Library at Strawberry Hill, watercolour original by John Carter 1784

Above is the original watercolour by John Carter showing the library at Strawberry Hill, published by Walpole in 1784. Below is a photo I took of part of the 1:6 scale set in progress, under natural light without the full decoration, just to rehearse how the first try-outs of the shelf books were going to look. In addition to the blocks of 4-5 books at a time I included a number of individual books which could lean against them and impart, I’d hoped, a less regimental, more informal and certainly less tightly packed look than most of the other ‘old library’ references I’d seen. The other reason was that there would be scenes where some of the books fell from the shelves and started flying around the room!

David Neat, set for stop-motion animation (in progress, unfinished) c 1:6 scale

To look more closely at the ‘singles’ first .. my plan for the more detailed individual books was to prototype a collection of different covers and ‘spines’ in various matching sizes, and assemble these around a Kapa-line foam core. This was because the books had to be as light as possible and it was also because I had a good technique for scraping the foam with rough sandpaper to look just like blocks of old paper. I had some sample swatches of embossed paper from the firm E.Becker and these, together with some vinyl wallpaper patterns, were just the thing for creating some variety in the book cover surfaces. I cut and sanded shapes in 2mm Palight foamed-PVC and spraymounted the patterned paper on. I sanded/impressed the ribbed spine parts in Kapa-line foam.

David Neat, props for stop-motion animation, book parts ready for mouldmaking, c 1:6 scale

David Neat, props for stop-motion animation, moulds and casts of book parts, c 1:6 scale

I think I must have run out of my usual Lukasil 429 silicone rubber to make all of the moulds so for the spines I used some leftover paste-form silicone which involved completing the mould block with a plaster ‘jacket’. The casts above are made from Tomps Fast Cast Polyurethane. Below is a collection of individual books ready for painting.

David Neat, props for stop-motion animation, cast books unpainted, c 1:6 scale

David Neat, props for stop-motion animation, various 1:6 scale model books

Above is a selection of the individually finished books showing the range of sizes and different treatments. There are touches of gold, which I preferred to be very sparing with. Thanks again to Astrid Baerndal http://www.baerndal.eu for the beautiful photo!

The bulk of the shelf books needed also needed to be as light as possible. Because of the size of the model and the number of shelves to be filled I think I’d calculated that it would involve about 5 metres worth of miniature frontage!. For these ‘blocks’ I shaped individual fronts (only about 2cm deep) varying the heights and thicknesses, stuck them together and made moulds from them. These Kapa-line prototypes below are already simply painted because I wanted to test whether the detailing would be sufficient when dry-brushed to look worn.

David Neat, props for stop-motion animation, casting runs of books, c 1:6 scale

David Neat, prototype and mould for 'book blocks'

Shown above is one of the block moulds together with, this time, the painted resin cast. What is visible at the bottom of this is actually the top .. I’d realised I would have to detail at least the first centimetre or so at the top because this might be seen. Below shows the making of these complete blocks in progress, involving a short line of ‘frontage’ with a ‘complete’ book attached either side. This was necessary because the full depth would be seen when the loose individual books in between fell or flew out.

David Neat, props for stop-motion animation, 'blocks' of shelf books being made

David Neat, props for stop-motion animation, book moulds being filled with resin and foam

The parts of these book-blocks were cast in a resin/Fillite mixture (Fillite is a very light, grey ash filler commonly used in resin casting, especially where reduced weight is needed). As a further reduction to the weight I inserted blocks of Kapa-line foam while casting.

I’d made the range of individual, more detailed books first so I could make moulds of some of these to cast the larger end-books for the blocks, because for these it didn’t matter that one side would be blank.

David Neat, props for stop-motion animation, completed books ready to be moulded for re-casting

As I’ve said, there were a few special books that either needed to be opened and read in the course of the action or others which would flap like birds around the room. Luckily for me, I didn’t need to introduce tight hinges to animate this ‘flapping’, so I choice to make the practical books using cut portions of cheap notebooks, choosing only those in which the pages were firmly glued to a cloth spine which I could also attach to the cast covers. I could seal most of these pages shut, leaving a few free at the place of opening. These I covered with copies of minutely scaled-down text on especially thin cream coloured paper.

David Neat, props for stop-motion animation, making a 'working' book, c 1:6 scale

David Neat, props for stop-motion animation, c 1:6 scale practical books

I had a particular challenge coming up with a method of achieving the elaborate, raised cobweb design on the main book above. I wanted it to be as fine and sharp as possible so this ruled out drawing it on with a relief medium, even one of the relatively fine relief outliners used in glass painting. In any case, this might not have survived much handling! Luckily I had been thinking for a while about possible methods of ‘working in negative’ .. that is, casting into voids or depressions made to achieve certain effects instead of working ‘positive’ .. so I made use of the ease with which Palight foamed-PVC can be finely incised (a little like lino-cutting) as a mould for casting this very detailed form.

David Neat, props for stop-motion animation, carving a 'negative' for raised decoration on a 1:6 scale book

 

Clear and reliable advice for working with silicone rubber

I’m sorry to say that the purpose of this short posting is not to ‘give it’ .. at least not yet .. rather, I’m turning the tables just for once and ‘asking for it’!

For example, if anyone can recommend sources of information they’ve relied upon in their practice I’d be interested, particularly in those that go into a little bit more detail or even those that are a bit more exploratory. I try to provide clear guidance here, but I haven’t really organized it all into one place yet, and I’m trying to get my facts ‘straighter’, and maybe pick up some more tips from others, before I do. The information provided by manufacturers should always be the first ‘ports of call’, and some suppliers do have some reliable ‘how to’s on their websites .. but maybe there are some I’ve missed along the way.

David Neat, mouldmaking and casting, Lukasil 429 and catalyst

I’d also be interested in what you have experienced regarding ‘shelf life’ i.e. how long you’ve continued using the components before the mix behaves differently to when you first got it? How did that compare to the supplier’s advice? Were you given a clear statement of shelf life in the first place, or did you have to hunt/gather to find it? Worst of all .. did you only find it written on the product once you’d got it? You see, it’s basically evident that suppliers or manufacturers show rather dubious behaviour when it comes to owning up to shelf life. Also, have you ever tried the ‘unthinkable’ i.e. tried a silicone rubber with a catalyst not intended for it? I’ve never had any catalyst going spare, and you probably haven’t either .. but if anyone has done, I’d be very interested in what happened?

David Neat, mouldmaking and casting, making a silicone mould in coloured layers

I recently did some tests which confirmed what I’d always suspected (or maybe read at some point in the past, because I see it is mentioned very occasionally) .. that however old the silicone ‘body’ might be, as long as it moves when you tip it, you can make it work by getting some new catalyst. I tested remainders of silicone which had been shoved in a corner waiting to be taken to the Recycling Centre, some going back more than 7 years .. and they all worked! Yes, they were different .. they were much more viscous than they should have been; the working time was much shorter, and they cured in no time at all! .. but still good for some things!

David Neat, mouldmaking and casting, making a skin mould on shaped styrofoam

When you’ve bought a silicone has it come with the essential facts i.e. not just the ratio of the two parts to mix, but the following: .. its weight in grams per cubic centimetre (good to know if you want to work out how much you’re buying in volume); its viscosity (useful when making a choice, because thinner ones could flow into detail better and leave fewer air bubbles); recommended cure time, etc. Has your supplier even said .. most importantly .. whether it is an ‘addition cure’ (platinum) or a ‘condensation cure’ (tin) silicone? This is important because whereas condensation cure silicones nowadays are fairly unaffected by anything, ‘addition cure’ silicones are a different matter. Or .. are they really? At times I’ve treated addition cure silicones really carelessly in the past without any problems. Again, any personal experiences would be welcome!

You see what I’ve done here? In asking the questions I’ve given information at the same time .. think of it as a standard teaching technique, though reversed in this case. Nevertheless .. I really do need other people’s help on this subject, so any facts you can contribute would be much appreciated!

More work with styrofoam

I’ve made some additions to my .. according to the statistical accounts .. most visited page Shaping Styrofoam which is under ‘Shaping’ in the Materials section. One is that epoxy resin glue works very well to bond it! I’d always assumed that epoxy would damage it, in the way polyester resin does .. but no, it doesn’t dissolve it and the bond is very strong! .. and I’ve used the cheapest stuff around, the one from Poundland! The other addition deals with preparing styrofoam prototypes for mouldmaking and I’m reproducing the entry here. I’m also finally managing, by the way, to hint more at what I’m up to at the moment .. working towards a solo exhibition of my current sculptural work which will take place in or around September next year!

If a styrofoam shape is being made as a prototype form intended for casting it doesn’t need to be made particularly durable .. it only needs to withstand silicone rubber being either brushed or poured over the surface. It does however need to be sealed, because if not the silicone rubber will grab into the surface too much and become very difficult to separate. Vaseline (petroleum jelly) is an ideal temporary sealant in this case because it can be easily brushed or rubbed into the micropores without damaging the surface. If care is taken not to use too much of it the Vaseline will also even out the surface, although I’ve noticed that most of it is absorbed into the silicone anyway. The only problem is .. it’s very difficult to see where you’re applying it! The solution is to colour it.

base unit shaped from styrofoam

This is one of many base-unit prototypes I’m making for a sculptural work which I can describe best by its working title .. ‘the ridiculously organic construction toy’! The components of the ‘nature driven’ form system will be assembled by means of holes and joining-plugs, hence all the holes in the base. Once I’ve made the mould from this the base units will be cast in polyurethane. I found a laughably easy way to carve out clean holes in styrofoam and I will explain this method sometime soon.

pigmented Vaseline

The best way to colour Vaseline is to first mix a little powder pigment, in this case half a teaspoonful, with roughly the same amount of Vaseline to make a thick paste not unlike tube oil paint. I chose the ultramarine here because it’s a strong pigment and finely ground, combining smoothly with the Vaseline .. some powder pigments may be grainy or clump a bit, which is not so good! The half teaspoonful was sufficient to give a strong colour to c. 50g of Vaseline when I added this to it, but one could use far less pigment. For example, the pigment will stain a porous prototype, so you have to bear this in mind if you want to keep it or if it’s an object of value.

using coloured Vaseline to seal styrofoam

There were a couple of larger scratches in the surface which I needed to fill and I’ve found that soft modelling wax (this one is the Terracotta Modelling Wax from Tiranti) is the easiest to use, worked carefully in with a brush.

filling larger holes with modelling wax

That’s actually it .. surprisingly short this time!

Making hollow casts in open or ‘closed’ moulds – Part 2

This follows on from the previous post in which I mentioned that hollow casts can be made in ‘closed’ moulds i.e. without having to set up a pouring hole. The cast is achieved in exactly the same way as the puppet head .. by building up a sufficiently thick layer in both halves of the mould, then joining them together. In fact it can even be a little easier since the two mould halves often have a consistent rim to work up to. This method of casting is a big advantage when the prototype form offers no convenient area for setting up a pouring hole, as is the case with the form below.

silicone rubber mould of a light bulb

Chloe Allen moulded this lightbulb while participating in our Modelling, mouldmaking and casting course in 2012. She wanted to preserve the distinctive shape of the contacts at the base, which would have to be remodelled if this area were cut out to form a pouring hole.

light bulb casts in Sculptamold and PU resin

The cast on the right, which came out near-perfectly, was a thin shell casting using polyurethane resin (Biresin G26 in this case) and Fillite. Note how finely the silicone rubber and resin have captured the smoothness of the glass. When silicone rubber is used on glass the surface must be very thinly greased with Vaseline to prevent the silicone from sticking. The distinctly different result on the left was obtained using Sculptamold as a casting material. Sculptamold is a mix of casting plaster and cellulose fibres, bought ready-mixed in dry form, making a thick paste when water is added. It feels and looks very similar to papier-mâché pulp, except that it sets hard in about the same time as regular casting plaster i.e. around 30mins. One has to work fairly quickly and paste the mix into both halves of the mould to form a thick shell. The paste is very workable and has good thixotropic properties, meaning that it is non-slump. When working with polyurethane resin the shell can be left fairly thin at the rim, because this will be strengthened when more resin is rotated around the closed mould. But Sculptamold is too thick to do this with .. instead the walls of the cast need to be built up to a good thickness right to the rim, but preferably with the top edge sloping down towards the centre of the mould, so that the mould halves will close properly when they’re put together. Usually once this is done the Sculptamold has already set firm, the top edge can be trimmed with a knife if need be and excess Sculptamold cleaned away from the mould surfaces. A little more needs to be mixed and then ‘piped’ or troweled on one or both of those edges before the two mould halves are joined together. Since the edges were sloping downwards most of the fresh Sculptamold will be pushed towards the centre of the mould, though a little will be squeezed the other way and will form a thin flashing on the cast which can be easily removed.

Because of its mix of plaster and fibres Sculptamold traps a lot of air, and this is impossible to get rid of, resulting in the surface effect shown above. Although the plaster component becomes firm very quickly the fibres retain moisture so, like traditional papier-mâché pulp, the material needs many days (weeks even! .. for thicknesses over half an inch) to dry out completely. After the two mould halves have been pressed together the setup should be left at least for a couple of hours before the cast can be safely demoulded. Even then the damp Sculptamold surface is somewhat fragile, rather like slip-cast clay, but this can be an advantage because it means that the mould seam can be more easily cleaned up, even using sponge and water to blend it a little if need be.

A while ago I wanted to reproduce two interesting fragments of driftwood I’d found on the Thames shore (we live only a couple of hundred metres away). The one below was a fairly complicated form which I knew would be difficult as a poured cast because of air entrapment, plus the fact that I didn’t want to compromise any detail of the form by cutting out a pouring hole.

Thames driftwood hollow resin casts

Thames driftwood hollow casts

In each case the original driftwood is on the right, the resin copy on the left. The paintwork isn’t complete in these photos .. just a basecoat with the first, lighter dry-brushed colour over it. I’d scrubbed the casts with warm water and Cif to give a slight ‘key’ to the resin surface and to remove any remaining greasiness. I used Rosco SuperSaturated acrylic, a theatre/film scenic paint which dries especially matt and has a strong binder.

I made the silicone moulds in the usual way .. by embedding the form up to a half-way point all round in plasticine or wax; coating the first silicone half over that, followed by the first-half plaster jacket once the silicone was cured .. etc. See previous articles e.g. Making a supported silicone mould for a life-size head .. for details of this method. Again as usual I coated the two mould halves with a polyurethane resin/Fillite mix, building up a strong shell.

silicone rubber mould of driftwood

As with the mould above for the smaller of the two forms, it can get tricky to determine in some places where the object surface ends and the mould seam surface begins, especially if some parts of the object are flat and thin. Although I’m only thinking of this now, and am yet to try it .. it could help if the very first, thin layer of silicone on the object, the detail coat, is coloured differently from the rest to make the border of the form clearer. Silicone rubber will accept a small amount of powder pigment to colour it without affecting its properties, as I illustrate in the next example.

showing resin edge cleaned up

As always with this method, the edge of the resin shell needs to be cleaned up so that there’s nothing preventing the two silicone mould edges from fitting together. If in doubt or if the mould halves no longer meet properly when testing them together, it’s better to shave away a bit more than necessary. Usually the resulting gap in the shell is filled when the final batch of liquid resin is rotated around inside the closed mould. In the failed example below, either the resin/Fillite mix was a little too thick with Fillite to begin with or I’d waited too long before pouring into, closing or rotating the mould. The resin hadn’t travelled enough along the whole seam line.

faults in hollow casting

Here’s a brief account of making a similar mould .. at least, done for the same reasons .. but with some differences in the method. Once again, I used these found objects mainly as test pieces .. this time tackling a dryer ball. The original below is pink and the cream coloured one is the cast. Fairly obviously this form presents only one option for making a poured cast .. setting up a pouring hole in the space where the writing is. But I wanted to keep the writing, and in any case .. managing to fill all these little ‘horns’ without trapping air would be impossible without the assistance of a vacuum chamber to pull the air out. I don’t deal with these more commercial methods because most people, like myself, are unlikely to have one.

Dryer ball original and cast

I also wanted to try covering with a complete silicone layer first, without embedding the form and applying it in two halves as I’d mostly done before. I intended to cover it and then split the silicone skin with a scalpel. The problem though would be finding the right place to cut once the form is covered .. ideally I needed to cut around the middle exactly in between the rows, where the manufacturer’s seam line is. I thought I’d solved the problem with the following, but it didn’t work out as cleanly as hoped.

covering half-way with silicone rubber

I applied the silicone rubber layer in two stages .. the first one above, and once this had cured, I completed with the second half below. It doesn’t make any difference if it is applied in sections like this .. the second section will fuse completely with the first. My idea here was to colour the second half to make the cutting line along the mid-point clearer. This is ordinary, not especially finely ground powder pigment, used in theatre scenic painting. The best way to mix with the silicone is to combine it thoroughly with a very small amount of silicone first .. to wet it in other words .. before adding more silicone. I’ve found that powder pigment blends very readily with silicone rubber. Apparently up to 10% powder pigment by weight can be added to silicone without affecting its properties. For this I used roughly 3g Ultramarine for 40g silicone rubber.

Applying coloured second half

Below is the containment setup I made around the mould using modelling wax, in order to make the first half of the rigid mould jacket, also called the mother mould. This enclosing jacket is necessary, especially when making larger moulds, to keep the flexible silicone skin in its proper shape. There are more, step-by-step photos showing how to model this containment at the end of Modelling wax in the Materials section.

setting up for mould jacket_5

The mould jacket can easily be made using a hard casting plaster, especially if this is left for a while to thicken up so that it can be troweled on over the form. For this though I decided to use Jesmonite .. which is basically the same as plaster but using an acrylic polymer liquid in place of water. This makes the material much tougher, and if a reinforcement such as jute scrim is also used, a stronger but thinner shell can be made.

mixing Jesmonite

Jesmonite powder and polymer liquid can be mixed together in a ratio of anything between 3:1 to 2:1 powder to liquid dependant on the pouring consistency needed. 3:1 gives a thicker mix and is more economical since the powder (basically just a fine casting plaster) is by far the cheaper of the two. Contrary to the way plaster is normally mixed .. the polymer liquid should be added to the powder. This should be thoroughly and vigorously mixed until the consistency is even. This is possible by hand for small amounts but the manufacturer of Jesmonite recommends using a special power drill attachment for mixing larger quantities.

1st pour for mould jacket

Above, I have poured the first small batch of Jesmonite over the mould form and to fill the ring around it. Below, I’ve started pasting small pieces of jute scrim into the wet Jesmonite. More can then be mixed up to cover the scrim .. and the procedure can be repeated to build up a strong shell.  I used two layers of scrim for this small form, but one would probably have been enough .. even for much larger mould jackets such as the one featured in Making a supported silicone mould for a life-size head .. I only used 2-3 layers.

layering with jute scrim

finished mould jacket half

The Jesmonite took very little time, less than 40mins, to set hard and shortly afterwards it was safe to remove all the wax and turn the form over. The silicone will eventually be sliced using the Jesmonite rim as a guide.

cleaned up first mould jacket half

But first, shown below, I’ve set up a wax wall for making the second half of the Jesmonite mould jacket. This is exactly the same procedure as before except that the Jesmonite rim needs to be thoroughly Vaselined to prevent the second half from sticking to it!

preparing for second half

Here is the completed mould being dismantled. I had thought that applying the silicone in two colours would indicate the line I had to cut in the silicone (i.e. between the rows of ‘horns’) clearly enough. But it wasn’t accurate enough, and in the end I got some parts of the horns on the seam line.

completed mould halves

It meant that these along the seam were much more difficult to fill, and my first tryout using polyurethane resin and Fillite didn’t work perfectly.

making the hollow cast

But for the second attempt I used a very thin, unfilled resin .. Tomps Fast Cast .. manually filling each ‘horn’ bit-by-bit, including a lot of jiggling around with cocktail sticks to dislodge trapped air. It was quite a lot of painstaking work .. but here again is the perfect cast, just to prove that it’s possible!

Dryer ball original and cast

Making hollow casts in open or ‘closed’ moulds – Part 1

This post follows directly from the last one in which I featured one of the simplest ways of making a complete mould for a puppet head .. making a 2-piece block mould in silicone rubber. At the end of the post I included a couple of photos of a hollow casting using filled polyurethane resin and now I want to explain how to do this in more detail. I will also deal in later posts with making hollow casts using other materials such as Jesmonite and the advantages of being able to make a hollow cast in a ‘closed’ mould .. i.e. without having to set up a pouring hole at the mouldmaking stage.

mould with casting

Jumping forward for the moment above and below, the hollow resin cast is almost finished and just needs a little cleaning up on the seam line. Polyurethane resin normally cures a white-to-beige colour dependent on the type and this cast is light grey because I added a filler called Fillite when mixing it. Fillers are added to resins for many different reasons (see Common fillers for resin casting in the Materials section) but in this case it is specifically to thicken the resin to help it stay put on sloping surfaces. Fillite also makes the resin easier to carve or sand without reducing its strength too much. Unlike polyester resins, there is no thixotropic or ‘gelling’ additive available for polyurethane resins.

casting nearly finished

For this test piece I used Fast Cast Polyurethane Resin from Tomps (see Quick view comparisons of casting materials for current prices) which is particularly thin to begin with, but the slower version has the advantage of a slightly longer working time and the ‘turning’, i.e. when the resin changes from liquid to solid, is not so abrupt. Below is the equipment needed for correct mixing. Polyurethane resins come in two equal parts, almost always mixed 1:1 by weight but in the past I’ve often got by without any problems by judging equal volumes in two disposable plastic cups, even though the weights of the two parts are slightly different. Now that I’m even more grown-up I prefer to measure properly by weight, using a fairly inexpensive kitchen weighing scales. Because the cans or bottles resin comes in are never designed to assist the pouring of small amounts .. the manufacturers would prefer that we use it all up in one go! .. I always decant some of each part into plastic cups and then pour from those when measuring. To avoid knocking these over while working, I made the cup-holder shown out of foamed Pvc.

materials for hollow resin casting

I usually mix Fillite with resin in the proportion 1:2 .. that is, equal amounts by weight of all three parts. This is easiest to remember and it also usually results in a thick sludge which is still easily spreadable and which will still manage to fill fine detail. It’s best to mix the Fillite thoroughly into part ‘A’ of the resin first (which is the ‘resin’ part of most polyurethane brands). The resin will combine with the Fillite surprisingly smoothly, to form a thick paste, which of course becomes thinner and more manageable when part ‘B’, the hardener, is added. Mixing must then be both thorough .. and fast! .. but shouldn’t need more than 30 seconds or so for small amounts like this. I usually mix up 10g part ‘A’, 10g Fillite and then 10g part ‘B’. The best mixing sticks I’ve found are disposable chopsticks because they’re very resilient, clean easily and can be re-used.

mixing Fillite with resin

The best practice is to pour most or all of it into the open mould-half straight away and sway the mould to let the slushy liquid cover the surface naturally first. In my experience it’s rare that air gets trapped with this method and using this mix, but if you’re concerned about deep detail there’s always a bit of time to poke around with a cocktail stick or small brush to make sure air is freed. An alternative is to take the extra time to brush on a thin detail coat all over first and let this firm up before pouring in more. For a few minutes the resin will pool back into the centre, but I work round the mould with a soft but rigid brush (synthetic is good) pulling it back up the sides for as long as I can until it starts to change. I try not to take it over the mould edge i.e. the outline of the form, but it doesn’t matter if this happens because this line can be cleaned up before the resin is fully hardened. Obviously, with the mould I’m featuring the neck part is completely open .. I had to edge the mixture very carefully into this part at first, but as it congeals it’s easier to build up a thickness.

close-up of mould being filled

After about 5mins or so (though this will vary with different resins) it can no longer be distributed so easily with the brush .. and it’s very important to stop trying at this point! .. because in doing so one risks separating the now gum like resin from the mould surface. In this state it’s possible though to press it, almost model it, with the fingers. Also at this stage if you want to use the brush again you need to clean it quickly in acetone.

filled halves of mould

I never try to do the two halves of the mould at once, however simple the form is .. most often the quick curing of the resin doesn’t give enough time for this. It is inevitable that the cast is much thicker in the deeper parts, but I’ve always found that if I follow the procedure described even the thin sections end up strong enough. They’ll get an extra covering during the next stage anyway. Resins have a so-called green stage (polyurethane having a longer one than polyester) when the thinner sections of the resin remain quite flexible. This can be taken advantage of .. let’s say it’s been 20-30mins since pouring .. for going round the mould edge with a scalpel or fine wooden modelling tool and peeling away anything that’s crept over the line. If not, the mould halves won’t fit together tightly! Now that they’re ready, the two halves of the hollow cast are going to be joined together from the inside .. by closing the two mould halves together and pouring in just enough resin to fill the seams.

Vaseline between mould halves

Above, I am brushing Vaseline on the remaining silicone surface, but being careful not to get it on the resin .. not so clear in the photo above. Vaselining the ‘seam faces’ (that is, the parts of the mould which come together to form the seam) is not an essential move when using silicone, but I’ve found that it often helps a lot! It provides an extra seal which halts the seepage of resin (this time mixed with a little less Fillite, to make it more liquid) out of the mould. I’ve also found that the Vaseline helps the silicone halves to align better.

For this internal coating I mixed up a small amount of resin and Fillite in the same way as before, but this time 10g part ‘A’, just 7g Fillite and 10g part ‘B’. I poured most of the mixture immediately into one half of the mould, placed the other half of the mould on top, made sure that the mould was secure and then rotated the mould carefully along the axis of the seam line to concentrate resin in this area. Obviously I had to be careful not to tip too far when running the resin close to the open neck part. Basically one has to continue in this fashion, ‘see-sawing’ around the whole seam and back, until one’s fairly certain that the resin has become too thick to move much more. Here it helps to have some of the resin remaining in the cup, as shown below, to indicate how thick it’s become. Another option, for those who have a little less patience, is to accelerate the curing with heat. Below I’ve set up a small heat gun to blow into the mould. I’m holding the mould because it needs to be moved around .. if left static it would get too hot. Once there’s no more obvious movement of the resin I usually leave the mould alone for a while, only demoulding the form once the extra resin in the cup is completely hard to the touch. If you don’t have this as a control, it should generally be safe to demould 1 hour after pouring, whichever brand of polyurethane resin you’re using.

heating resin to quicken curing

Obviously the advantage of being able to make a hollow cast like this is that it is lightweight, while still being strong. It also saves on material. If strength is of particular importance, more than one coating of resin can be applied or strengtheners such as glassfibre matting or scrim can be integrated into the two halves before the mould is put together. This is not a method of speedy mass-production .. it takes considerably more time than pouring a cast .. but manually ‘applying’ the cast in sections, as it were, does ensure that you can make perfect, blemish-free casts every time. If for any reason a solid cast is preferred, it’s easy to fill the hollow casting with more resin .. although it may be better to do this in stages for forms larger than this one because the heat produced by larger amounts of resin could cause tensions during curing which have been known to crack the casting.

removing flashing with scalpel

As I’ve said in the previous post, there’s always a seam to be cleaned up .. but in this event the work was minimal. With polyurethane resin the flashing (as the excess is called) is particularly easy to remove .. but trimming and sanding is made even easier by the addition of Fillite. My preferred method is to scrape with a scalpel, in the direction away from the blade edge, because I find this easier to control.

 

 

Making a simple 2-piece silicone block mould for a puppet head

Here I am using work photos I took of a simple 2-piece block mould I made some years ago to illustrate the basics of the mouldmaking process. There’s more than one way to make a mould for any given object and more than one choice of material to use. Actually the options, both for the material used and the type of mould, are not so many though, they can be counted on one or both hands .. but this is enough to make the decision a considered one. The choice depends on the level of accuracy and detail-reproduction you want; the material and shape of the prototype; the type of casts you want to make; the amount of money you can afford to spend and the amount of time you’re prepared to put into it .. see ‘quick view’ comparisons of mouldmaking materials in the Materials section.

carved puppet head

The prototype here is a carved, wooden puppet head measuring altogether 10cm long. It has simple features and the surface detail is not especially fine or intricate. It was painted and given a satin varnish as a seal. The parts which will need more attention to during the process are the protruding chin, nose and the ears. A 3D head like this can be ‘divided up’, so to speak, for mouldmaking in a number of ways, illustrated below.

different ways to divide a head form

If a mould in two halves is planned, a line around the object needs to be found which would then allow the two pieces of mould, once completed, to be pulled away relatively easily in opposite directions from the object. The two parts of a head mould are most often divided like the form on the left above .. a line bisecting the neck which travels up, usually following the outer edge of the ear, to meet at the top of the head.I say ‘relatively’ easily because using a flexible mouldmaking material such as silicone rubber does allow a certain amount being in the way .. in other words undercutting .. because the mould will flex and stretch over obstructions, at least to a certain degree. The extent to which the silicone is expected to do this will determine whether you use a silicone that is particularly soft and flexible i.e. with a low Shore A value, or one which is tougher with a higher Shore A value. At least, that’s the idea. In actual fact I think that most mouldmakers, like myself, settle upon using a tried-and-trusted general-purpose silicone rubber with a medium Shore A to manage most things.

Setting up is probably the most important, time-consuming and exacting part of the mouldmaking process. By ‘setting up’ I mean preparing the prototype to receive the mouldmaking material .. something which often involves separate stages, each beginning with their own set-up.

embedding the prototype

Above is the completion of the major part of the set-up for the first mould half which in this case involves embedding the prototype up to a half-way mark in soft modelling material. I’ve already cut the plasticine bed to a shape, the reasons for which I’ll come to later. At the time when I was making this mould I used standard plasticine for this bed (in the UK the ‘Newplast’ type in long cellophaned packets). This was cheap, re-usable and easy to work with. Nowadays I prefer to use soft modelling wax, which is all of the above but easier to clean up (if needed) from the surfaces of prototypes and moulds. See the ‘worklog’ in Modelling wax in the Materials for an account of using it for mouldmaking set-ups.

But for this one, plasticine was packed fairly loosely around the puppet head first just to secure it in place on the baseboard and then built up to reach the half-way line. Although the plasticine must form a clean, smooth and ‘watertight’ seal where it meets the prototype form, the top surface around this doesn’t need to be clinically smooth and usually won’t be flat since it has to undulate around the form. In fact the less flat the better to some extent, so that when the two silicone halves take this form they will lock together better. That’s the reason for the spaced depressions I’ve made in the surface with the smooth end of a paintbrush. The silicone poured on top will fill these marks and the second silicone half will reproduce them, and that will help these halves to lock together in exactly the right place. It’s important though that the initial marks made are just enough to be seen i.e. not too deep, because otherwise the natches (as they’re called) created in the silicone may catch when trying to put the mould halves together prior to casting.

Since I’d decided to divide it straight down the middle the half-way point was fairly clear so I didn’t need to mark it on the prototype first. But one has to mark it somehow for more complicated or less symmetrical forms, and after scratching my head about the best way to do this without defacing the prototype I came up with what I think is the best solution. Below I’ve rolled a thin string of plasticine and am using it, since it can be repositioned, to rehearse possible dividing lines. Once the right one is found this can usually be kept in place when embedding.

marking dividing line

The next stage, below, is making a sturdy containment wall into which the first silicone half can be poured. I could have just cut a strip of cardboard to the right height, bent it round and pressed it firmly into the plasticine, for example. But I chose to slice the plasticine bed straight down in a shape first, because I wanted to re-use the cardboard wall for both halves of the mould. I made the cardboard strip to fit around the shape, scoring on the outside to help it bend cleanly in the right places. I’ve sliced the plasticine away completely at the neck base so that when the mould halves are made it is left as a pouring hole. I’ve made the cardboard strip high enough to extend approx. 1.5cm above the highest point of the prototype, to make sure that the silicone block would be thick enough here (in total it should be high enough for both halves of the mould if it is to be used again later, see below) . At the time I didn’t take photos illustrating the final details before mixing and pouring the silicone .. taping the wall securely in place; making sure there were no gaps, including smoothing the plasticine edge to meet the cardboard, and Vaselining the inside of the cardboard wall to help it release cleanly from the silicone .. but you can get the idea.

making containment wall

The next photo I took jumps ahead to dismantling the first mould half once the silicone had cured i.e. once it had hardened properly. It’s not exactly necessary to illustrate how to mix silicone rubber .. but there are a few things to say about choosing, mixing up and pouring it!

In the first place, there are different hardnesses available indicated, as I’ve said, by the Shore A number. Because I was making this 2-piece mould just in silicone and unsupported I chose one which was a little tougher than I would usually for a small form .. T30 from Tiranti in London, which has a Shore A of 30. I judged that if the block as a whole was thick enough it would have the strength to be cast into on its own, but still be flexible enough for some slight undercutting i.e. in this case, the ears.

T30 has to be mixed with 2.5% of its catalyst to cure properly, and it’s best to be as accurate as one can. The catalyst supplied is designed to be mixed by weight rather than volume (this is the case with almost all silicone rubbers, and many resins). This means that digital weighing scales are needed, but nothing special .. inexpensive kitchen scales are fine (see the equipment list in ‘Beginner’s Basics’ – mouldmaking and casting explained). But before one can portion out by weight, one needs to know roughly the volume .. I needed to at least estimate the amount of silicone I would need to fill the containment for the first half, partly because silicone is too expensive to waste. The best way to measure the volume needed is to fill the containment with something which can be easily tipped out and measured .. I’ve done it many times now with rice, for example. This needs to be done before the mould setup is Vaselined, for obvious reasons! I found that I needed 260 cubic centimetres (or ml) of fluid silicone to fill the first half. One could then just measure out 260ml of silicone rubber in a calibrated mixing pot (another item that’s often essential for this work) and whatever the weight comes to needs 2.5% added in weight of catalyst .. or one could multiply 260 by the SG (the ‘specific gravity’ which is the weight of 1 cubic centimetre) which is included on the packaging info, to get the exact weight needed and therefore not need a calibrated pot.

Once catalyst is added to the silicone the chemical reaction  .. the curing .. will start but with most silicone rubbers there is plenty of time to mix properly and pour. This should not be hurried .. it needs to be thorough! I use strong, straight-sided, translucent polyethylene beakers and disposable chopsticks for stirring. The bamboo chopsticks are very resilient .. they need to be because silicone rubbers are quite viscous. It is important not to forget either the bottom or the sides of the beaker while mixing. As a ‘rule of thumb’ I make sure that I spend at least 5mins mixing. Nowadays I prefer to use a silicone with a pigmented catalyst .. when there are no more streaks visible you know when to stop .. but T30 has a colourless catalyst. According to Tiranti T30 has a 20mins pot life i.e. the amount of time for mixing, pouring or applying it before it starts to change. This is short compared to many silicones but it also has a shorter curing time of 8hours, compared to the more usual 24hours.

If the prototype form has a lot of fine surface detail, particularly recesses, it’s usually worth brushing the surface with a coat of the catalysed silicone first to make sure that all detail is filled. Because the puppet head didn’t have much surface detail I just started pouring the silicone slowly into one corner of the containment, letting it flow round the form and gradually rise at its own pace. This is quite a slow pace, but patience is necessary because otherwise air can become trapped.

I swear that many years ago .. I mean about 20 .. when I first started using silicone rubber I read something about it needing a certain amount of moisture in the atmosphere to assist curing, and I always used to get better results putting moulds in the bathroom while curing, perhaps even with a wetted flannel or two placed near them. I don’t think this is necessary anymore because silicones have developed, but I still do this in the summer if the air is particularly dry. What’s important though is that you put the moulds in an undisturbed place with a level surface for curing.

Below, after about 8hours (recommended for T30) I dismantled the mould containment carefully, making sure that I could use the wall again.

dismantling setup for first half

To make the second mould half the whole setup needs to be turned over and, firstly, all of the plasticine (or whatever is used for a temporary bed) must be removed. This is usually straightforward and can be quick, as long as one’s been mindful not to pack the modelling material too tightly all over the prototype in the first place.

plasticine to be removed

I had some cleaning up to do where the plasticine had stuck. Most can be removed with a wooden modelling tool or a cocktail stick, but if need be plasticine can be dissolved either with Vaseline or white spirit. I think one has to resist the temptation at this stage to take the prototype out of the mould. It would make cleaning much easier, but there have been rare occasions when I’ve tried to put the prototype back after cleaning and it hasn’t sat properly .. i.e. slight gaps visible that weren’t there before .. because some detail on the prototype was catching.

plasticine residue

first half cleaned

I had made sure that the cardboard wall was cut to twice the height I had needed for the first half, so that when re-assembled it was already the right height for the second mould half. Now it’s really just a repeat of everything done for the first half from this point .. with one very important extra! Do not forget that now you are pouring silicone onto silicone, which will fuse completely unless the cured silicone surface is greased with Vaseline as a barrier! As I say, just greasing the surface, no more than that. Since Vaseline is transparent it is difficult to check whether you’ve been thorough. I usually go over it with the brush 2-3 times to make sure (not adding more Vaseline, just moving it around) or .. I’ve done this occasionally .. put a little pigment in the Vaseline to make it visible.

setup for second mould half

Silicone rubber moulds can be used as soon as the recommended curing time has passed. A tough rubber such as T30 should last many years and could deliver hundreds of casts if using non-aggressive materials such as plaster, Sculptamold or Jesmonite. Far fewer perfect casts are achievable with resins, because over time resins such as polyurethane or polyester will alter the silicone, making the surface more brittle .. small bits of surface detail can break off eventually.

completed block mould

Being able to make successful castings is a separate subject in itself .. there are many different materials and as many ways of casting with them! But in order not to just leave it at that, here are photos of a lightweight, hollow casting I’ve recently tried using filled resin (polyurethane resin with Fillite).

mould with casting

If you do a lot of casting you will know that there is always a seam where a little of the casting material has seeped into the join between the two silicone halves, but if all has gone well this can be the easiest thing to clean up.

casting nearly finished

In the next, or the next-but-one, post I will detail this method of making hollow casts using thickened polyurethane resin, which has the advantage that ‘enclosed’ moulds can be made and filled without the need for a pouring hole .. in other words no alteration to the complete form of the prototype. This present post will be copied as a page in the Methods section under Mouldmaking/casting as part of my plan to consolidate this information in one place.

For current suppliers and prices of silicone rubbers see ‘quick view’ comparisons of mouldmaking materials under mouldmaking in the Materials section. This will also give an overview of the other properties or advantages of silicone rubber, as well as the drawbacks.