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

 

Small props for stop-motion animation

Back in 2011 I was making settings and props for stop-motion animation, and one particular scene I’d been asked to work on involved the eating of an apple down to its core. The film called for a kind of poetic realism .. I mean that its world was ‘ours’ to an extent, the puppets were recognisably human though stylised, furniture and props needed to be fairly accurate and believable, even though the action was often dreamlike. This was one of those moments in dream when reality is tugged a little more into focus, so although a loose approximation of an apple getting smaller in bite-shaped chunks could have sufficed I wanted to make the moment properly convincing.

So I modelled the whole apple first in Super Sculpey .. in two sizes because one would be needed for close-ups and the smaller for scene shots. I made silicone moulds from these, and then enough casts for about ten successive bites of the apple. My intention was to carve away each bite in the sequence, so I cast in thin Fast Cast polyurethane resin mixed with a third of Fillite (a light ash filler) which would make the material nicely carvable especially if using a Dremel.

I guess I knew from the beginning, or at least pretty soon after, that I would have to manually copy the ‘bites’ on each successive one down the sequence, but I didn’t want to make more than one mould for each apple size. I made the stalks individually out of white styrene rod, slightly carved and sanded (and the ends ‘crunched’ with something heavy). I used Vallejo acrylics to build up a nice glowing red in layers, and kept the surface detailing to a minimum since each stage would have to be exactly copied.

I made a very simple mould for both using my usual Lukasil 429 (from specialplasters.co.uk, a silicone rubber I’ve been using for years which has always been easy and reliable). For small and basic shapes such as these it was enough to tack them with a little more Sculpey to a baseboard, build a containment wall around them and pour silicone as a one-piece block around them. Once cured the silicone needs only to be lightly split with a scalpel to take the prototypes out and make the casts. This is what I often call a split-block mould. This is the easiest form of 3D casting, each cast needing just a little bit of clean-up work around the pouring hole.

Advice on making props etc. for stop-motion animation

This was an example of a special prop serving a visual sequence which had been properly thought through. In this case the way the prop would be used was very clear. This is not always so, partly because room has to be left for on-site decisions during animation and partly because it’s rarely possible to think of everything anyway, especially if props are commissioned at an early stage, long before animation begins.

In this situation it’s always good practice to allow for possible changes, and include these contingencies straight away at the making stage as long as they don’t lengthen the making time too much. One very good move, where there’s a choice, is to pick materials which are relatively easy to alter. Foamed PVC for example is very easy to cut and can be re-glued instantly using superglue. Another prudent habit is to keep parts which ‘may’ have to move separate until the last. As an example, even if something like a school-desk isn’t likely to be opened (according to the script .. and there should always be a script!)  it may be wise to keep the desk-top separate, and give the underside and the desk interior the same colour treatment as the rest just in case. On the other hand I would never go to the trouble of making working hinges for this kind of ‘what if’ because it’s often easier to animate a movable part like that just with a concoction of Blu Tack and bent wire.

If you’re asked to make props or furniture for someone else’s stop-motion animation you can only work as efficiently as the information you’re given .. or, more truthfully .. the information you’ve had the sense to ask for! I’ve never worked on anything where I didn’t have to tease out important facts by asking a lot of searching questions. You will of course want the principle design directives first .. the scale or dimensions, and the full visual appearance of each article. Then, just as importantly, you will need to know details of how each is used if at all, or whether they are just background dressing. These are the main questions, but there are many others that one may not think to ask at first, so here are just some of them.

If a prop is going to be used in the action, do we see the puppet holding it? If so, how easily can the puppet do this? For example, does the prop need to be specially light? Do holes need to be drilled in the prop to attach fixing wires, or if something like Blu Tack or ‘sticky wax’ needs to be used is the paintwork suitably resistant? In the case of pieces of furniture, do they need to be secured to the baseboard (partly to keep their position, but especially if sat in or leant against)? If so, legs usually need to be fitted with strong wires or bolts at the bottom.

Has the question of ‘relative size’ been properly considered when deciding upon the scale of a prop? To put it simply, just like dolls or cartoon characters puppets often have larger heads and hands in relation to their bodies and their overall height. If, for example, a retro style desk telephone is needed and this is scaled faithfully according to overall puppet height, it may look reasonable enough in the background but if ‘used’ the speaker/receiver part may look ridiculously small against the puppet’s hand or ear! The solution might have to be that two differently scaled versions are made, or just one slightly larger speaker/receiver part.

If you’re proud of your own work, if you’ve taken good photos and want to publicize what you can do, will anyone object if you do this before the film itself has become public? It’s important as a courtesy to reach an agreement, even if it’s not something dealt with in your contract .. or even if there isn’t a contract! You should consider the fact that an independent stop-motion film may be many years in the making and this is a long time, either to not be able to promote your own work or to feel a bit secretive or guilty when you do. Often this can be resolved, as I’ve done in this article, just by not mentioning the film by name.

 

 

 

Finally getting the hang of Instagram

 

I’ve been thinking about tackling Instagram for a while .. because I desperately needed more opportunity not to have to write that much .. if you can believe it!  But because I process photos quite ‘seriously’ on my PC before letting them loose, and because I have a Windows phone which I’m determined to keep until the bitter end .. there just didn’t seem much hope! But recently I did yet another search for alternative ways of uploading to Instagram, and finally I’ve found a way that works .. like a dream! It simply involves installing the free browser Vivaldi on the PC (no need to make it the default browser) and accessing Instagram through that! The extra piece of software doesn’t weigh the computer down like some other methods I’ve tried and failed with. The only drawback is that it only allows me to upload one photo at a time rather than grouped .. but this could change, and personally I prefer that anyway. Here’s the link where I found out about it ..

https://www.techradar.com/uk/how-to/upload-photos-to-instagram-from-a-pc

I’ll be posting on Instagram more regularly than here, I would imagine. WordPress will remain my serious ‘writing’ place, and I’ll be able to elaborate here especially regarding ‘instructional’ content but .. you might have noticed .. I seem to have less and less time to do that these days. Here are some images from the couple of posts I’ve put on Instagram so far .. and if you’re interested, have a look at

https://www.instagram.com/davidmeredithneat/?hl=en

 

Above .. works in progress. Green styrofoam ‘beasts’ shaped in two halves, ready to be sealed to make moulds and casts from, and polyurethane resin cast ‘Arpish Dancers’ which I’m testing on a mock-lacquer sushi plate.

Below .. I recently ‘re-vamped’ some pen drawings from the Thames Foreshore made a few years ago, converting them into transparent ‘layers’, colouring them in Procreate and finishing them in PaintShop Pro. This is ‘Base#1-1’ and below is an enlarged detail from ‘Base#2-1’

Below .. resin cast ‘eggs’ and foamed-PVC ‘twigs’ collection. I’ve given the PVC my usual treatment of ‘graining’ with sandpaper and staining with Spectrum Noir alcohol ink pens, to resemble bone or wood. I’ve surfaced the mat underneath with a laminated digital sketch .. part of my experimentation with different presentations, or ‘contexts’ as I call them, for the ensembles of small sculptural forms.

 

Coating styrofoam with polyurethane resin

These are the latest forms I’ve been making for my .. not-quite-working-title .. Ridiculously Organic Construction Toy. For this I’ve been creating simulations of eroded rock and driftwood cast in resin, twisted Pvc branches covered in fake moss and lichen, corals, leaf clusters and strands of seaweed made from latex etc. But I also wanted to include some play elements which are more obviously scaled down, such as these brickwork ruin pieces. The best way of picturing the whole idea is to think of aquarium or reptile tank accessories and then imagine getting a large collection of these instead of a box of Lego. I’m still working on the question of how exactly the ‘construction’ is achieved .. i.e. how such components will be fixed together when playing .. but as part of the system I’m working on an artificial ‘mud’ which I’m hoping will solve part of it.

ruin fragments in resin-coated styrofoam

The forms above were cut/carved in regular blue styrofoam, textured using a heavy-duty wire brush and then coated in polyurethane resin. There’s a bit more to the ‘painting’ process .. something new I haven’t tried before .. but I’ll come to that. If properly done the method of resin coating makes the forms unbelievably strong! .. perhaps not enough to survive little children, but certainly any adult wear-and-tear.

Making a brickwork arch in styrofoam

These two photos illustrate other forms intended for the collection and the process of making them. I’ve described this method of form-making in more detail in Shaping styrofoam. The arch piece above started with a Pvc template, which I used to help sand a block shape. I found I had to make a separate drawing template (the one at the bottom) just in order to inscribe the brick pattern onto the styrofoam shape. Then I used the special diamond needle files pictured to scratch out the brickwork divisions at the right thickness. I wanted these pieces to be 1:12, i.e. usual dollshouse scale, but I’ll eventually use a mixture of scales.

Making a brickwork niche in styrofoam

To make the ruined ‘niche’ shapes above I also used the method I described in Shaping styrofoam of using a curved sander to create the concaves. I roughed out very deep channels for the mortar lines, because these will become partially filled with coloured resin .. and this is what gives the pieces unusual strength. I found it was better to make all the channels before attacking with the wire brush, because I made the pitted texture mainly by hitting or pressing with the brush. This peppers the foam with deep holes and it may fragment a bit too much if the channels are made afterwards.

diamond needle files

Here is a close-up of the type of file I’ve found to work best for detailing foam. These have a ‘diamond coated’ surface which has more of an effect on relatively soft materials than the other, cheaper, form of needle file which is just ‘toothed’, grooved metal.

wire brushes useful for texturing rigid foam

I usually use the smaller brushes pictured above when working with the more delicate polyurethane foam in Kapa-line foamboard, but styrofoam has a tougher surface .. the heavier wire brush has more effect. Importantly, the action in this case is not a brushing or sweeping one, it’s more hitting downwards and rocking around .. I call it ‘scumbling’.

styrofoam 'ruin' fragments

Now to get to the main point of this article! Of the polyurethane resins I most often use (Sika’s Biresin G26 and Tomps’ Fast Cast) I know that both can be used in the following way, but Tomps Fast Cast is best because it’s a little thinner, powder pigment mixes better into it, and according to Tomps it is designed to cure properly in very small amounts or in very thin layers. This is not the case with all polyurethane resins. I’m basically making a very quick-setting paint with it, and because it’s quick-setting it has to be done a little at a time. To dose both resin parts I use disposable plastic pipettes (which are available from a few places online) and usually work with not more than 2ml of each part at a time. I can normally manage to use up to 4ml before it thickens too much. Because there’s usually no time spare to clean the palette surface before it sets I use a ceramic tile which can be scraped clean afterwards. There’s always just enough time to clean the brush though, and this can be quickly done with acetone.

Coating styrofoam with polyurethane resin and pigment

Here I’ve dosed 1ml of each resin part together on the tile, added a small amount of powder pigment, mixed the whole together with a synthetic-hair paintbrush and used the same brush to paint the foam. Synthetic is best because the hairs will be rigid enough to push the pigmented resin into deep pattern, but full and fine enough to hold a lot of the paint. Powder pigment is the best form of colour to use .. strong colour, inexpensive, available .. and I usually find that it mixes better into resin than it does with water!

The polyurethane resin has no effect on styrofoam (unlike polyester resin), it will cure hard and ‘fused’ to the surface, and it’s done .. that is, it’s touch-hard and ready for further work .. in about 15 minutes! Whereas regular paint such as acrylic will infiltrate more and contract as it dries, polyurethane resin does less of both so there will be a little ‘smoothing over’ of fine surface detail. It will also be a gloss finish! .. which I don’t like, would never choose, and at the moment I’m experimenting with the different  ways of dealing with this. There is no matting additive for polyurethane resin, and regardless of which pigment or filler is mixed with it, the top surface exposed to the air will always be glossy. Obviously painting over with another matte paint, such as a good acrylic, is an option .. but polyurethane needs a lot of preparation if the paint coat is to resist a lot of handling and this is made difficult by such a patterned/textured surface.

One possible solution is to use my own version of cold powder coating. If you google ‘powder coating’ you will find that this refers to an industrial painting process in which fine thermoplastic powder is melted onto metal to create a durable surface. It’s very like the enamelling that you might have done at school, with coloured glass powder on a copper plate, melted in a small oven. My version does not require heat, and it’s perhaps more related to the model-making practice of scattering granules into glue to create a surface .. but it does share some of the surprising durability of these other methods!

crushed brick

Below is a close-up of the styrofoam ‘ruin’ forms after coating. I first gave the bare styrofoam an undercoating of resin mixed with black pigment, and then a second coat without any pigment, covering a small area at a time. While each portion was still wet I sprinkled a mix of finely crushed brick and sand onto the resin. I’m fortunate in that, living close to the Thames beach, I can pick up fragments of any colour of brick, illustrated above. Since these have already been broken down by the elements they are much easier to crush to a powder using mortar and pestle.

detail of brickwork surface done with 'powder coating' method

While working I could see that the particles were readily sinking into the thin coating of resin, and when the excess is shaken off after a few minutes the powdery top layer still adheres strongly. Polyurethane resin is a strong adhesive, especially if the dust or particles are porous and jagged. Having tested the strength of the surface once the resin cured I have little doubt that it is permanent. I still have to do some paint finishing on these pieces, emphasizing contrasts and colours and giving more ‘speckle’, but I have no worries about regular acrylic paint attaching itself on top. The greatest bonus in this particular case is that these pieces have a lot of the look and feel of real brick .. because that’s what it is!

 

Using plaster as a filler in polyurethane resin

I’ve been asked this question a number of times now .. whether regular plaster can be used as a filler for resin, in place of the other ‘white’ powders more commonly used such as talc, chalk dust, marble dust etc. Don’t forget that these are all versions of calcium carbonate and are chemically inert, whereas plaster is calcium sulphate and certainly not as ‘inert’ since it reacts so strongly with water. I hadn’t ever considered it as a filler, and hadn’t heard of any cases of it being used in regular practice. My advice up to now had therefore been to avoid it, because I assumed that it could affect the curing of resins. Plaster is hygroscopic meaning that it will readily absorb moisture from the atmosphere however well it might be stored. Powder pigments are the same, and I have found that the slight moisture in them will cause polyurethane to foam and expand a little even when just a little pigment .. i.e. up to 10% by weight of resin .. is added. So I always assumed that adding a more substantial amount of plaster would cause bigger problems .. not only affecting cure but probably also thickening the resin too much to pour properly. Yesterday I finally found the time to do some tests using a couple of regular plasters with polyurethane resin and discovered that although there are some adverse effects these could also be turned into benefits.

Expansion of polyurethane resin when filled with casting plaster

For the first test above I made a control mix of Tomps Fast Cast polyurethane resin without any filler .. 15g of each part, so 30g total. The mix set touch-hard in just a few minutes as normal, becoming a pale ivory solid (the cup on the left). I then did the same but added an equal weight (30g) of Crystacal R which is a fine, hard, ‘alpha’ casting plaster. As per usual with polyurethane resin, the whole amount of filler has to be mixed thoroughly with Part A before adding the hardener Part B.

Whereas mixing is usually very smooth using conventional fillers such as Fillite, the plaster/resin needed a lot more stirring before the lumps disappeared. But after some effort the two combined making a smooth but thick liquid .. like treacle. As usual though, this thins down quite a bit once Part B is added, and the resultant mix was still very pourable. Far from the reaction being slowed down by the plaster I found that the cup started to get warm very quickly, and then the liquid started to expand. Once it had set touch-hard it had practically doubled its volume, as shown by the initial mark I’d made on the cup. The mass was solid, hard and ‘dry’ within 30mins .. there was no under-curing, failure to mix or greasiness on the surface .. all the indications of a good cure!

comparison of volume of 30g unfilled resin with 30g plus plaster filler

What was completely unexpected was the change in tone .. from the normal clean, pale ivory to something slightly darker, dirtier as shown here .. and I can’t really explain that yet! The test piece detached cleanly from the cup and the surface was smooth as shown below. The only indication of foaming was minute but noticeable pocking of the surface towards the top, none at the bottom.

I measured the volume increase compared to the control pour, both before and after foaming. The control pour measured 30ml in volume once solid, roughly consistent with the SG (‘specific gravity’ or weight per ml) of the combined resin parts given by the manufacturer as 1.1g. The volume of the same amount of resin with 30g of Crystacal R added .. before expansion .. was just 40ml. This is also consistent with the way plaster behaves in water, absorbing much of the liquid volume. The volume of the expanded mass once set touch-hard was 70ml.

effects of foaming visible on cast surface

I expected a roughly similar result when I repeated the test using the same proportions with pottery plaster in place of Crystacal R  .. but the result was more dramatic! Pottery plaster is a coarser, softer-setting ‘beta’ plaster, called ‘pottery’ plaster because it’s designed for making the absorbent plaster moulds ideal for slip casting. In the first place whereas the 30g Crystacal had combined with the 15g Part A resin eventually as a smooth liquid, the same amount of pottery plaster became a thixotropic paste rather like car body filler. Addition of Part B thinned it considerably but it was still a significantly thicker liquid than that obtained using the Crystacal.

expansion of polyurethane resin when filled with 'pottery' plaster

But more importantly, foaming was more ‘aggressive’ producing larger bubbles and until it set hard the mixture expanded to almost three times its original volume. As before though the mass became solid and hard within 30mins with no tackiness or other evidence of failing to cure.

larger-scale foaming on surface using 'pottery' plaster

However, as shown below there was more noticeable damage to the cast surface in the topmost area because the bubbles here had become much larger. As with the Crystacal the volume of 30g resin combined with 30g pottery plaster prior to reaction was 40ml, but this expanded to 115ml before setting firm.

larger-scale foaming visible on cast surface

I made a sectional cut through the upper parts of both test pieces and sanded the surface .. the fine casting plaster to the left and the pottery plaster to the right below.

cut sections showing foam structure

I can only account for some of this marked difference in behaviour. The fact that the pottery plaster appeared to thicken the mix more is predictable .. it is because of the shape of the particles. Commercial fillers such as Fillite are composed of minute microspheres which roll over each other meaning that quite a lot can be added to a liquid without affecting its flow too much. The particles of pottery plaster must be jagged, causing them to clump together whereas those of the Crystacal must be finer and smoother. As for the stronger foaming reaction and increased expansion .. the pottery plaster was older than the Crystacal and may have acquired more moisture; it may also contain an additive; or it could have something to do with the particle size. I’m not entirely sure!

I imagined though that whereas plaster would never be a sensible option for flawless casting, these results could have some uses. At the moment I’m making cast versions of pieces of driftwood to use as components in a sculptural project. So far I’ve been hollow-casting these in polyurethane resin, but using Fillite as a thickener. I’ve described this casting process in my article Making hollow casts in open or closed moulds in the ‘Methods’ section under ‘Mouldmaking and casting’. One difficulty with this technique is getting a thick enough build-up, especially on vertical surfaces, when using polyurethane resin because there is no way of making it truly thixotropic. I tried the pottery plaster/resin mix for coating these moulds below, with a little black pigment added. I found that because of the swelling it was much easier and quicker to build up a thick shell, even on the vertical parts.

making a hollow cast in polyurethane resin

I also found, as I’d observed from the cup tests, that since the foaming is largely directed upwards there was no damage or loss of detail on the cast surfaces. The intricate patterns of weathered wood have reproduced perfectly here!

hollow casts using filled polyurethane resin

 

 

Casting materials

I’ve updated prices, suppliers and added proper entries for principal materials in the casting section under Materials above. The ‘quick view’ comparisons page provides an overview for anyone not sure which of the various casting materials to use, while the other pages give more detailed information on the properties of each and how to work with them. Not all are there yet in detail, I am slowly working through them .. but so far the pages include polyurethane resin, polyester resin, polyurethane foam and polymer-modified plaster.

These more detailed pages begin with a summary ‘definition’; outline what the material is best at and not so good for; describe their ‘working life’ i.e. how to use them and how long for, and lastly what they cost and where to get them. At the end is a ‘worklog’ where I can add other bits of information as I have it.

Here is an excerpt from my page dealing with polyurethane foam. It’s the first one in this section to include photos, but I hope to do the same with the others.

Working life

Polyurethane liquids generally have a recommended shelf life of under a year, but I recently made a test with this flexible polyurethane foam bought at the end of 2012  .. so, more than two years old .. and it worked perfectly! For more advice on disregarding ‘shelf life’ have a look at the start of the ‘quick view’ comparisons page in this section.

self-skinning flexible polyurethane foam, old batch still usable

I always write the date or period when I buy materials, plus reminders if anything needs special handling .. as with part ‘B’ of the mixture here which needs to be shaken because the ingredients separate after it has been standing for some time. This is common with resins especially those that are pre-filled. Another thing .. not only common, inevitable .. is that the cap or lid for the ‘hardener’ component (usually part ‘B’) gets stuck because traces of the liquid crystallize. Something which has never failed me so far even with the most stubborn screw-caps is a strip of tough rubber to grip around the cap while turning.

strip of rubber to help unscrewing caps or lids

As with resins, polyurethane foams consist of two component liquids which are mixed together in a set proportion by weight, so having a good digital kitchen scales is essential. With this one from Tiranti the ratio is 2:1 part ‘A’ to part ‘B’. But in addition to being able to weigh accurately one also needs to judge the volume in this case, to be able to dose the right volume of liquid needed to completely fill the mould when it expands.

For example if the expanding foam has to fill a volume of 1,000 cubic centimetres (equivalent to a 10cm cube) and the foam is expected to expand up to 5-6 times its original liquid volume, then dividing 1,000 by for example 4.5 should ensure that the mould is filled, with a little surplus. This gives us close to 222 ml of liquid needed, of which two-thirds of the weight is part ‘A’ and one-third part ‘B’. So let’s say we need 148ml of part ‘A’ and 74ml of part ‘B’. We must now find out what these would weigh. Luckily the SG (specific gravity, written as the weight in grams of 1 cubic centimetre of ml of the substance) is often given on the containers. If not it will be on the MSDS (Material Safety Data Sheet) available online from the manufacturer or supplier. The SG of part ‘A’ of this expanding foam is 1.05 so 148ml would weigh 155.4 grams, and the SG of part ‘B’ is 1.13 so 74ml would weigh 83.62 grams. We need to round these figures off a bit but also adjust them back to a 2:1 ratio .. 156 grams of part ‘A’ to 78 grams of part ‘B’.

Below, I didn’t need to make a specific volume calculation in this case because I was just testing whether the material still functioned normally. I poured an arbitrary amount of part ‘A’ .. 17 grams .. into the cup first, then 10 grams of part ‘B’, a little more than half the amount. I did this because I’ve come to expect that with ‘old’ materials it’s the catalyst that’s most often affected, becoming weaker.

part 'A' polyurethane foam being weighed

polyurethane foam part 'B' added 2:1 by weight

Above, 10 grams of part ‘B’ has been added. It is always important to think ahead when preparing for this work! Make sure that you have all your necessary tools etc. to hand .. i.e. as here, a mixing stick .. so that you don’t have to hunt around for them at short notice. As soon as the part ‘B’ is added it should be quickly stirred in because the reaction will start within a few seconds. If the mixture needs to be decanted into a form mixing should not be more than c. 10 seconds before transferring it. I’ve tried mixing the material directly in the mould form a few times in the past but this has often resulted in an uneven result with parts not properly curing.

2-part polyurethane foaming within 10 minutes

The foam will have fully risen within about 5-10 minutes. Out of interest I calculated exactly how much it had done so in this case. There was 17g of part ‘A’ .. so 17.85ml in volume using the above calculation, and 10g of part ‘B’ being 11.3ml in volume .. altogether 29.15ml. The foam rose to fill the cup with a little more on top so by measuring water in the cup and adding a little I estimated 210ml. So the polyurethane had actually expanded to 7.2 times its original volume! I don’t know whether the increased expansion was due to the age of the material or the fact that I added a touch more part ‘B’ .. but it’s worth experimenting with!

When I bought the foam in 2012 it was for making these forms shown below (I’ve placed an old casting in its mould) .. and I recorded at the time that the polyurethane only expanded 4.5 times its volume. The mould is plaster-jacketed silicone rubber (made a long time ago when I was living in Hamburg and I can’t remember why I used a translucent silicone). The silicone doesn’t need any release agent against the foam but plaster certainly will if you want to keep it clean .. and Vaseline will be fine. The surface of the cast didn’t achieve the smoothness of the mould .. but one shouldn’t expect it to.

form cast in flexible polyurethane foam, showing silicone rubber mould

What this photo shows well is how much polyurethane discolours over time. In this case it was just discolouration though .. after three years exposure the feel of the surface was just the same.

freshly foamed and 3-year old polurethane cast compared

Test 19/12/2012  72g (48g part A to 24g part B) expanded to fill the ‘Koerper’ mould with just a little pushing out of the top, which cut then be cut off. The volume of the ‘Koerper’ form was measured as 325 ml so expansion was 4.5 times (weight to volume). The foam took c. 25mins to reach tack-free curing

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