P – Q

PAINT

All paints are likely to contain the following ingredients:

Pigment which determines the colour. This is most often a dry mineral or synthetically produced powder which is usually very finely ground.

Solvent whether water, a mineral spirit, oil or other synthetic liquid in which the pigment is dissolved and the evaporation of which effects hardening of the paint

Binder the adhesive medium which holds the other constituents of the paint together before and after drying. The drying or changing of this will work in conjunction with the evaporation of the solvent to achieve the hardening of the paint.

Many paints will also contain one or more fillers. These are principally meant to extend the volume of the paint without diminishing the intensity of the colour (though in many cases they do). But they also have the more important function of making paint more opaque, if this is desired.

Most paints undergo ‘drying’ i.e. hardening by evaporation of the solvent (and the solvents in the binder) and most achieve a resistant surface after full drying. Some paints, like acrylic, dry from the outside inwards, achieving a touch-dry skin while underneath is still liquid. Others such as enamel dry in the opposite way. But with all paints it is important to understand that although a paint may be firm and dry to the touch very quickly, proper durable hardening will take longer, at least a day or often a few days even with so-called ‘quick-drying’ paints. Artist’s oil paint will need considerably longer.

PALIGHT (FOAMED PVC) SHEET

‘Palight’ is a brand of foamed Pvc sheet, the softest and easiest to work with for model-making purposes. See page ‘Palight brand foamed Pvc’ in ‘constructing’ in the ‘Materials’ section for an illustrated account. See also the later post ‘Does foamed Pvc have a grain?’ from July 18 2014 for utilising this property when working with this or other extruded plastics.

Soft, matt white plastic (also some colours and gloss version, principally for 3mm or 5mm thicknesses). Very light-weight! Foamed interior makes it easy to cut, but has excellent rigidity and resists warping. Flexibility makes it very durable. The standard matt white version is available in thicknesses from 1-19mm.1mm ideal for fine cutting and delicate construction, especially suited for delicate work because it gives slightly under the knife and has little obstructive grain. Superglue must be used to bond (though Pvc ‘pipe-weld’ is an alternative which also allows for some repositioning). Other solvent glues such as UHU may latch temporarily on the surface but the bond is not strong. Palight can be painted with anything if primed first (best to give light coat of Simoniz acrylic car primer). Palight can also be sanded down, carved or shaped, even embossed, unlike card. The Palight brand is softest and easiest to work with of all the brands of foamed Pvc (others go by the names of Foamex, Forex etc .. although these tend to change over time). 2mm upwards is supplied with protective film on one side (which one must remember to remove!) Foamed Pvc is manufactured mainly for signage i.e. large, weather-resistant billboards or retail display purposes.

Obtainable info on prices and suppliers is kept updated either in ‘Quick view materials info’ in ‘constructing’ in the ‘Materials’ section, or in the ‘Suppliers’ section in ‘Updated sources/prices for specific materials’

Price comparison with cardboard At current (2013) Bay Plastics price for 2mm Palight the equivalent of an A1 sheet would cost c. £1.80 before delivery costs. Good cardboard, i.e. Mountboard, can generally cost £3-5 per A1 sheet

See www.palram.com the manufacturer of Palight, for specific and more technical product literature

Patination fluid for applying to a metal surface to create/accelerate natural corrosion effect i.e. ‘verdigris’ patination fluid is a cold corrosive which will turn copper surface the distinctive blue-green. Available e.g. www.jpennyltd.co.uk

Recipe/- 2parts white vinegar,1.5parts non-detergent ammonia, 0.5parts non-iodized salt. Spray onto surface and allow to dry 1 hour, then re-apply to missed areas. Allow to develop overnight. Salt ratio will affect shade i.e. more salt bolder green, less salt more greyish. http://moiracoon.blogspot.co.uk/2011/05/copper-patina-verdigris-recipe.html

Paverpol is the brand name for a tough-drying (PVA-like) paint formulation manufactured in the US mainly for the hobby market which is suitable for painting on fabrics or foams to give them a much tougher surface. Fabric will become rigid and almost ‘resin-hard’ if soaked with it and the manufacturer recommends it for outdoor sculpture. Obtainable can be found at some art shops but go to http://www.paverpol-uk.co.uk for more information

PAX paint see ‘Prosaide’

Petrobond an oil-bonded sand-based compound (available from mindsetsonline.co.uk) used for the sand casting method allowing quite fine reproduction of detail. It is a mixture of fine sand and clay, held together with mineral oil. See ‘sand casting’.

plastazote A flexible foam, self-skinned and mainly in sheet form, available in a wide variety of colours, densities and thicknesses. Can be firmly bonded together with EvoStik Impact adhesive, Dunlop Thixofix or (in some cases) superglue. Can be bent more with heat. Easy to cut with a sharp knife, cannot normally be sanded though. Obtainable Pentonville Rubber Co, 4D, some hobby or craft shops. Price varies

PLASTER

Plaster is a powder obtained by grinding and heat-drying gypsum (calcium sulphate) which when water is re-introduced sets by chemical reaction (the gypsum rebonding with water) to form a hard solid. ‘Casting plaster’ is usually higher grade, fine and strong e.g. ‘Basic Alpha’, ‘Prestia Expression’ or ‘Crystacal R’ good, reliable brands. These are ‘alpha’ plasters, likely to be much stronger than the ‘plaster of Paris’ commonly sold in chemists or hobby shops .. and a lot stronger than the plaster sold in builder’s suppliers for plastering walls. These casting plasters also often mix smoother and flow better. All plasters should be mixed by shaking plaster into water, never the reverse. Using acrylic polymer solution (instead of water) will make an even stronger casting material (mix is different, usually 3-parts plaster to 1-part polymer, see below). Plaster can be left for a brief while (a few minutes) after being added to water before being stirred to allow more air to escape, also for better mix

Advantages                                                                                                         Inexpensive; readily available; reliable; long shelf-life if properly stored; easy to mix (with care and practise); Health&Safety friendly, no smell; good range of brands with different properties (varying in hardness/fineness/porosity/setting time/mix viscosity etc.); hard plasters will accept certain fillers

Drawbacks                                                                                                                     Not as suitable as resins for small or delicate forms (too brittle); not as ‘free-flowing’ as resins (much higher viscosity); mixture can easily be uneven, leading to imperfections on cast surfaces; not as easy to patch as resins; when used for casting, more measures needed to avoid air bubbles e.g. detail coat where possible, using a surfactant; difficult to alter the appearance of plaster (i.e. using pigments or fillers) as much as one can with resin

Difference between ‘alpha’ and ‘beta’ plaster ‘Alpha’ plasters fine, hard, pourable, best for casting, more leeway in mixing/ ‘dental’ plasters hard, fast setting, may not pour so well/ ‘beta’ plasters soft, absorbent, better for sculpting/carving and essential for absorption moulds.

Often best to use fine casting plaster (alpha plaster) for most purposes e.g. making mould jackets (with recommended ratio 2.5-3parts plaster to 1part water). Usually buying small amounts (i.e. 1-5kg) plaster in art or hobby shops is most expensive option, always cheaper from specialist plaster suppliers.

Some plaster types compared

Basic Alpha £8.29 per 5kg (Tiranti). Very reliable, smooth pouring, fine casting plaster.

Prestia Expression casting plaster £20.08 per 25kg (specialplasters.co.uk) Fine, hard ‘alpha’ plaster. Mix by eye (rec. optimum mix 2.5-2.6kg per litre water). Water volume will constitute approx ½ final volume. Working time 8-10 mins. Can be demoulded min. 30mins after top water has been reabsorbed

Crystacal R casting plaster £23.33 per 25kg (specialplasters.co.uk) Similar to above but harder and stronger. Recommended mix 2.86kg per litre water, does not pour well (2.2-2.5 makes more pourable mix). Pot life 10+mins, setting 15-20mins, demould 30+mins

Crystacal AlphaK ‘one of the hardest’ ratio 4:1, £45.57 per 25kg (specialplasters.co.uk). SWI ‘ultra high strength casting plaster developed to replace resin in the production of fine detailed giftware’

Terracotta Cast ‘extremely hard, containing iron. Light terracotta colour. Castings are suitable for outdoors and will ‘rust’.  £75.31 per 25kg, £10.50 per 2.5kg (Tiranti)

Pottery plaster £9.60 per 25kg (specialplasters.co.uk) Softer ‘beta’ plaster. Porous, suitable for absorption moulds. Mix by eye, but rec. optimum mix 1.5kg per litre water. Not suitable for casting, too weak. Must not become contaminated with release agents i.e. Vaseline

Changing recommended mix ratio

The recommended ratio for Crystacal R is 2.86:1 plaster to water by weight. This may have maximum strength but it makes a thick mix, difficult to pour. 2.6:1 instead is much easier to pour and castings will still be sufficiently strong. The recommended mix ratio which comes with the manufacturer’s directions reflects an optimum in terms of material strength and under industrial conditions (where casting is machine-assisted) and may bear little relationship to individual workshop practice. The best way to determine the mix is to experiment in getting the consistency required for the job and judge the results in terms of strength. After getting acquainted with measured ratios it may be simpler thereafter to mix by eye.

Mixing plaster

Estimate roughly how much plaster needed first; water volume will constitute roughly half final volume of mix for plasters with ratio around 2.5kg:1l, but for softer plasters the volume increase will be more like 166% (.. or, generally for hard casting plasters 100ml of mix will need 100ml of plaster and 50ml of water); test recommended ratio (the mix for maximum strength) at least once first, then mix by eye; milk bottles cut down make excellent mixing pots (easy to squeeze for spout, easy to clean); mixing always by shaking plaster into water; always dispose of plaster once set, never down sink or toilet even in trace amounts. Wait until set then bin solid

Tips for correct mixing loosen plaster in storage container, use cup method (shaking from small plastic cup, knocked on rim of mixing vessel), sprinkle plaster evenly and steadily into water until water volume has been filled if judging by eye. Tamp constantly, let stand for at least a minute (can be more, plaster will only start to set once stirring takes place). Mix thoroughly for at least 3mins (this is often recommended by master mouldmakers). Try to pour in thin stream and in stages, rocking and tamping mould.

Top water must be fully reabsorbed and temp cool before demoulding. Generally plaster not suitable for thin castings (less than 10mm), though depends on size/shape of form. Plasters have a very generalized SG of roughly 1.2 g per ml.

Colouring the plaster mix For calculating beforehand how much pigment can be safely used, at max 10% by weight;- If 100ml of final mix is needed, this will require roughly 100ml of plaster and 50ml of water. Plaster has a generalized SG of 1.2 so the weight of the mixture will be 170g. So up to 17g of pigment can be added in theory before affecting setting.

Common problems

Within the freshly filled mould water collects at the top extremities but is then reabsorbed creating voids and runs. Solution: either prefilling these areas or overall ‘detail’ coat first, reducing water content in mix to pourable minimum, ‘mopping’ off excess water in mix with a tissue or sponge.

Mouldmaking with plaster

Absorption moulds (see entry) need a soft, porous plaster whereas plaster used for mould jackets needs to be strong as a relatively thin shell so hard ‘alpha’ plaster is more suitable. Plaster can be used as a direct mould in some cases (i.e. when casting in a flexible rubber or when designing multi-piece moulds for fibreglass work but in the case of the latter the plaster needs special sealing and ‘parting agents’ before the fibreglass is applied

PLASTER/POLYMER

‘Alpha’ plasters can be mixed with polymer liquid instead of water for a much stronger result i.e. for outdoor sculpture. This is often referred to as ‘polymer-modified plaster’ but probably best known in the UK packaged as the brand ‘Jesmonite’ although the two materials can easily be found separately.

Polymer liquid £9.25 per 1kg, £35.34 per 5kg (Tiranti, 2013). Makes plaster much stronger, enabling either finer castings, laminating (with matting or scrim) and exterior use. Only with ‘alpha’ plasters, generally 2.5-3:1 plaster:polymer by weight. In this case, contrary to general rule re regular plasters, the liquid is best added and mixed quickly into the plaster (like mixing cement). Measure out recommended ratios first. Thorough mixing essential. Up to 10% more polymer or water can be added to thin the mix. Marketed as a friendlier alternative to resin for strong castings but can’t compete with resins for casting thin or delicate forms (too viscous and not as strong). Will pour better than straight plaster and longer fluid pot life, but much more difficult to eliminate bubbles.

Manufacturers recommend always power-mixing but determined hand-mixing works fine for small amounts.

Jesmonite’ is similar (the two components sold together). Mix 2.5-3parts powder to 1part liquid. Jesmonite AC100 (general-purpose) £25.60 per 4kg kit (4D); 1kg liquid £9.72, 3kg powder £13.38, £60.00 per 20kg kit (canonbury arts) See also entry ‘Jesmonite’

Specialplasters.co.uk stock Acrylic Polymer SP201 ‘for adding to the water of plaster mix’ starting point 2:1 water to polymer. 1litre £5.28, 5litres £21.12

Plasticine Oil-based, non-drying clay. ‘Newplast’ most common brand in UK. Can be softened with heat but not melted. Can be made paintable and a little more durable by coating with PVA glue. The British version (Lewis ‘Newplast’) does not melt whereas the American Van Aken can be melted and poured. Plasticine can be cleaned off/dissolved/smoothed to a certain extent using Vaseline. Obtainable most art or craft shops Price around £1.80 for 500g

Notes in progress ..

There is understandably some confusion over names and similarities of products! This is not helped by the fact that the different manufacturers want to keep the exact recipes of their products secret. For example Becks Plastilin , manufactured in Germany www.becksplastilin.de  is composed of ‘micro-waxes’, paraffin oil and mineral fillers (such as chalk) and powdered pigments according to the website. They produce a range of children’s clays but also produce what they call an ‘industrial clay’ for professional shaping/modelling which is mid-grey in colour. The ‘Plastilin’ sold by Flints in London (at least the firm grey one) could be from Becks. Becks is certainly worth supporting because they make a point of employing people with special needs in their factory (see website videos).

On the other hand the original invented by Franz Kolb at the end of the 19th century in Germany is also called ‘Plastilin (full title ‘Muenchner Kuenstler Plastilin’) and still available today. Franz Kolb’s firm has now moved on to specialize in producing a range of non-drying clays more suited to machine prototyping (i.e. hard enough to be carved or routed to create shapes) and less so for hand modelling. This type of plasticine is commonly termed ‘styling clay’ and has been especially used in the automotive design industry for the styling of cars. A characteristic of these styling clays are their limited and neutral colours i.e. greys or browns. Kolb’s range includes sulphur and non-sulphur, clays with no oil content, specially elastic, and lighter-weight varieties. http://www.kolb-technology.com/

William Harbutt’s 1897 patent …

4D stocks ‘Plastiline’ Grade 50 grey, 55 ivory (‘clay powder mixed with oil and wax, 2 colours, 2 weights and 3 hardnesses’) £12.66 per kg. Softens under heat, hardens in fridge. Claims can be painted with acrylic.

Chavant produces a form of plasticine for professional shaping/modelling .. specifically non-sulphur, much firmer versions. Price (2013) £9.48 per 907g (Tiranti)

PLASTIC

General term for a range of mainly synthetic, manufactured materials which share certain properties i.e. can be shaped when soft, retain a degree of flexibility (rarely brittle) when hard, a consistent composition throughout. Truer definition more in terms of molecular structure i.e. composed of polymer (long chain) molecules. In the case of plastics this often takes the form of a long ‘backbone’ chain (principally carbon) onto which a variety of other molecules can be attached. This results in the wide range of different plastics and their different properties.

Most plastics are derived from petrochemicals (the chemical products derived from petroleum) and therefore largely carbon and hydrogen in different arrangements.

As a making-material; more delicacy, rigidity, durability and general versatility is offered by plastic. Some cost less than equivalent card e.g. thin sheet styrene.

Difference between ‘thermoplastics’ and ‘thermosetting’ plastics This is the most important distinction between the two groups of plastics. Thermoplastics are meltable, i.e. they can be reformed repeatedly just be heating them and this will not change their chemical composition. Thermosetting plastics cannot be remelted after their initial formation

See for further info ‘Palight foamed Pvc’, ‘Styrene’, ‘acrylic plastic’, ‘ABS’, ‘ASA’, ‘Acetate’, ‘HIPS’, ‘Polypropylene’

plasticizer an ingredient which softens a compound or makes it more pliable. For example many soft modelling materials, especially polymer clays, contain plasticizers

Platsil Gel is the brand name of a very soft, translucent silicone rubber (from the company Polytek and distributed by Mouldlife in the UK) suitable for prosthetics and animation puppets. See post ‘Flexible puppet hands’ April 19th 2012 and also entry ‘prosthetic silicones’ here

plug in mouldmaking is the usual term used for the part of a mould which is made to fill out a large void such as the open mouth in a lion’s head or the area underneath a four-legged animal. See post ‘January mouldmaking exercises – ‘Two legs good ..’ Parts 1 and 2, beginning January 29th 2012

PLYWOOD

Thinnest ply at 4D is 0.4mm (grade3 Finnish birch) e.g. 304x304mm c.£7.75 (2012)

POLYESTER RESIN

An inexpensive 2-part resin which is usually either translucent or clear, and which readily accepts appropriate colourants or fillers. When cured it is hard, though brittle compared to epoxy or polyurethane unless reinforced.

Polyester resin is available in a number of different forms, the principal being the ‘GP’ (general purpose) resin commonly used for fibreglass work. GP resin is translucent with a slightly beige/brown tint. The next most familiar is the ‘clear casting’ version which cures glass-clear and colourless and is often used for the embedding of objects in clear blocks. There is also ‘gel coat’ resin which is pre-thickened, and opaque white resins (pre-pigmented) are also common. All are catalysed by mixing a measured amount of the same hardener (containing MEKP methyl ethyl ketone peroxide) ranging from c. 1-4% by weight.

GP polyester resin is often referred to as a ‘laminating’ resin ( i.e. specially for fibreglass work) and the ‘clear’ often referred to as a ‘casting’ resin. Although GP can be used for casting, it is not common to use the clear polyester for laminating partly because it is more expensive.

For more detailed information including how to work with it, suppliers and prices see the entry under casting in the Materials section.

polyester coating  Because the foam inside Kapa-line foamboard is polyurethane it will accept resins like polyester (whereas styrofoam dissolves). GP (general purpose) polyester resin from Tiranti £9.36 per 1kg, catalyst £2.42 per 50g. Mix in very small (10g) amounts and catalyse at 2% (i.e. 4-5 drops per 10g of resin). Coat using soft brush, careful re hair loss. Working time is c. 14mins in warm room, to 18mins in cooler room.  Average setting time 20mins, ready for light sanding after a few hours. Forms better left outside to set if conditions are relatively dry. When brushed on foam surface it will partially soak in which means that one coating will produce a reasonably thick, strong shell when the resin is fully cured. Another coating can be added for greater strength but this may start to fill in detail.

polyester body filler for car repair comprising ‘body’ part and ‘hardener’ part. Difficult hardener to dose (not easily measurable). Sometimes useful as a paste for quick ‘press’ casting or for gluing or repairs. Average working time 2-3mins, cure time 20mins. Obtainable garage or car accessory shops e.g. Halfords; some DIY stores. Price e.g. £12 per kg average

’99p store’ sells small tubes ‘Body Filler’ from Mean machine 60g

POLYFILLA

‘Polyfilla’ has become the generalized name used in the UK for any water-based, gap-filling paste normally used for filling cracks in wall plaster. In the US the term used is ‘spackle’ or ‘spackling paste’. The first thing to understand is that these terms are generalized; there is no definitive ‘polyfilla’ or ‘spackle’, there are many different types with different ingredients and properties.

However they generally all have certain things in common; their main body is a white mineral powder, they include a binder which is usually a polymer of some form, and they contain or need the addition of water.

For example the MSDS for Polycell Trade ‘Polyfilla All-Purpose Powder’ lists 50-75% calcium sulphate hemihydrate (which is regular plaster) and 10-25% ‘cement’ as the body powders with 1-2.5% vinyl acetate copolymer as the binder. The water is added as needed. On the other hand the ingredients of Polycell Trade ‘Polyfilla Fine Surface’ are listed in an older MSDS (2001) as 2.5% ‘talc, magnesium silicate’ and 25-50% calcium carbonate as the body, with vinyl acetate copolymer. Newer MSDS (2008) doesn’t provide that information (only lists vinyl acetate copolymer).

Polycell ‘Fine Surface’ filler I highly recommend this particular brand because it sticks well to any surface even when applied very thin (ideal for fine stippling). Will not crumble, dries with minimal shrinking, some flexibility, workable longer than others. It is more expensive than more standard ‘polyfillas’ such as Polycell ‘Multi purpose’ or B&Q ‘Value’ filler but worth it for its versatility (i.e. can also be thinned as a relief paint or even as a gesso and will even stick well to metal or plastic when diluted!). Ready-mixed. Thin coats are fairly quick-drying especially if thinned with water because then less likely to form a skin. Imm thicknesss takes roughly 1 hour plus. Obtainable B&Q, Leyland, other d.i.y or home decoration stores Price (2013) £6.48 for 500g tub, £8.98 for 1kg tub (B&Q) £8.29 for 1kg tub (Wickes)

polymer clay is a relatively new type of modelling clay made up of Pvc particles held together with binders. When heated the binders evaporate and the Pvc particles fuse to form a permanently hard solid. Polymer clays are usually very smooth to model with and allow fine detail. Commonly used brands include Sculpey, Fimo and Cernit

See ‘Super Sculpey’ for further info

polymer-modified plaster see ‘plaster/polymer’

Polymorph  Low-melt (60degreesC) plastic supplied in granules, can be press-cast. Boiling water or even strong hair dryer enough to melt it. Very tough once cooled, with some flex. Re-meltable, ideal as material for press-casting into or over a form but difficult to model with Price £20.00 per kg (mindsetsonline), £25.90 per kg (4D), £18 per 500g (Maplins)

Tip I find it easiest to use a heat gun to melt. Putting a small handful of granules into ceramic or glass bowl (otherwise they’ll be blown away), heating until all turn transparent, waiting until cooled a little then kneeding mass together, reheating and kneeding again if necessary.

polypropylene is a plastic familiar to most as the translucent material of plastic milk bottles etc. It can be bought as sheets in different thicknesses (0.5 – 1.5mm), most commonly with a ‘frosted’ appearance (although clear is also available). It is also made in different tints i.e. yellow. blue, green etc and sheets in A4 size can often be found in art/graphic shops. It is a very useful material to use in model-making either for simulating frosted windows or glass panels or to suggest scenic gauzes in theatre design models. It is easy to cut with a scalpel, being relatively soft and flexible, but difficult to glue since even superglue can’t adhere to it (hence superglue bottles are often made from this or a similar plastic). Double-sided tape will work though for holding it in position. Obtainable 4D modelshop, London Graphic Centre, Paperchase (main store in Tottenham Court Rd)

polystyrene more properly called ‘expanded polystyrene’, familiar as a very light-weight packaging material and ceiling tiles, with a coarse-grained ‘cellular’ surface. In the US this is known as ‘styrofoam’. Obtainable Wickes sheets 1200×2.4mx25mm £6.99, 1200×2.4mx50mm £13.49, 1200×2.4mx75mm £20.99 (2013)
Other available forms/- as ‘veneer’ in rolls 50cm x 10m (Wickes £4.99) and as granules for filling cushions.

polyurethane foam exists in a number of different forms. As the interior foam in Kapa-line foamboard it can be easily carved, sanded or embossed. It is also available as a liquid, in two parts, which will result in a rapidly expanding foam when mixed together. There is a version which sets rigid or another which remains flexible like ‘cushion’ foam. There are some other types of rigid polyurethane foam available in sheet or block form, but not common.

2-part expanding polyurethane foam This is supplied as two liquids which are quickly mixed together in a certain proportion and the mix immediately poured into a mould (which is then usually closed). The liquid expands rapidly to fill the mould and the foam sets to create a foam casting of the object. There are rigid-setting and flexible-setting versions available, also ‘self-skinning’ meaning that the foam forms its own skin on the surface inside the mould. It is usually white but can be coloured by adding pigment (mixing into one of the parts first)

Tiranti’s Self Skinning Flexible PU Foam (2013) £38.88 per 3kg Mix Parts A and B in ratio 2:1 by weight. Expands 5-6 times volume. 5mins to rise, 15-20mins demould. White cure. H&S care taken against breathing in Part B ‘harmful by inhalation and contact with eyes’ 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. 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

Tiranti’s Rigid Foam 002 (2013) £35.08 per 2kg Tiranti. 2 parts mixed 1:1 by weight. Expands up to 8 times, tack free in 10mins and demould in 15-45mins. This can be used as a fill material for hollow castings, giving extra strength without adding to weight.

POLYURETHANE  RESIN

Because of its ease of use polyurethane resin is the most common choice for small-scale casting and home craft-work. It is commonly opaque and fast-setting (usually 5-15mins, ready to demould 20-25mins) and mixed in two equal parts (easy to measure). It is generally less brittle than polyester resin and much easier to trim, sand or carve after setting, but also more expensive. Some types of polyurethane resin are significantly thinner than polyester and therefore a better choice for casting small, delicate or detailed forms. Opaque polyurethane resin doesn’t involve quite the same ‘health & safety’ measures necessary when working with polyester .. except that the transparent versions of polyurethane resin are much more hazardous and should be strictly avoided!

For more detailed information on polyurethane resin including how to work with it, different types, suppliers and prices see the entry under casting in the Materials section.

polyurethane rubber is noted as tougher than silicone (esp. re concrete casting and resistance to abrasion) with longer library life, but many disadvantages i.e. sensitive to moisture, shorter shelf life, bad release properties, often more expensive

pot life the amount of time one has to work with a material such as resin or silicone rubber once mixed

press casting is the method of creating a cast or imprint by pressing a soft material into a harder (usually inflexible) mould from which it can then be removed

Prestia Expression is the brand name for a hard, fine, casting plaster from Lafarge, available in the UK from specialplasters.co.uk

priming is usually necessary before painting non-porous surfaces (such as metal or plastic) to give the paint something to grip on. It is also often necessary to prime or seal even porous surfaces when using certain paints. For example if oil paints are applied to an absorbent surface (such as bare canvas fabric) the drying oils will be sucked too quickly from the paint preventing it from hardening properly.On the whole this doesn’t happen with acrylic paints, which can be applied to unprimed surfaces. Gesso, however, is commonly used as priming undercoat for both oil or acrylic painting.

Acrylic gesso/- e.g Spectrum Acrylic Gesso Primer, has intentional ‘tooth’, dries very slightly silky (needs 24hrs) 250ml £9.20, 1litre £18 (UAL). Spectrum also supply ‘Covent Garden Primer’, water-based for priming plastics. 250ml £4.55, 1litre £14.20 www.spectrumoil.com Their ‘Acrylic Primer’ has less tooth. Chromacryl Gesso primer, less tooth. Both contain latex.

Priming cardboard with an oil or spirit-based undercoat (or spraying with Simoniz car paint primer) will minimise warping when painted. Simoniz available matt white, red oxide or grey Obtainable 4D, Halford’s, many garages Price c.£4-6 for a 500ml can

Filler primers e.g. Hycote are thicker spray formulations designed to fill and smooth out minute surface imperfections while priming.

Prosaide is a special acrylic adhesive used for sticking latex prosthetics to the skin. It has become the vital ingredient of so-called ‘Pax paint’ used in SFX and stop-motion puppet fabrication enabling acrylics to adhere to latex and also flex with it. Regular acrylic is thoroughly mixed with an amount of Prosaide (usually 40-50%) and then painted on the latex surface. Obtainable henna-boy.co.uk (cheapest) or other online SFX makeup suppliers e.g. 150ml £14.25 after VAT+delivery from henna-boy

PROSTHETIC SILICONES

The type of silicone rubber which cures very soft (i.e. low on the Shore A scale) suitable for film SFX prosthetics and flexible puppets used in stop-motion animation. These silicones are usually translucent  with a Shore A of 10 or less. They can be made even softer with additives such as ‘Smith’s Deadener‘ in the case of the Polytek ‘Platsil’ range or a ‘slacker’ in the case of the SmoothOn range (see below). These soft silicones are usually ‘addition cure’ so mixing is easier and curing is faster, but they are subject to the vulnerabilities to contamination that addition cure silicones share (see ‘addition cure’). They are also usually more expensive than the regular mouldmaking silicones. The other properties that addition cure silicones generally share are high tear-strength and very minimal shrinkage (i.e. 0.1% as opposed to 1% with some condensation cure silicones).

Prosthetic silicones can be pigmented easily in-the-mix (small amounts of acrylic and oil paint tested on Platsil Gel 10) but are much more difficult to surface-paint. Tests were done (May 2013) painting cured Platsil Gel 10 by mixing up a very small amount of the silicone, thinning it with regular white spirit and adding acrylic paint. Unless a very high proportion of thinner is used the ‘paint’ remains very thick and difficult to work with. The surface then takes much longer to cure/dry than regular Platsil Gel, remaining tacky for a good few days. Just prior to complete drying the surface can be dusted with talc to matte down the glossiness. Specialized paints for SFX work are available but these are usually expensive (Smooth-On ‘Psycho Paints’, FuseFX paints).

Platsil Gel 10 (from Polytek) has a c.6min working time and 30mins demould. The ‘10’ denotes the Shore A. Parts can be mixed by weight or volume (since they weigh much the same). Mixes to a very thick liquid, cloudy/translucent, easier to brush into a mould than pour (thinning additive available from mouldlife.net). A retarder for extending the working time is also available from mouldlife.net, but putting in fridge also extends a little. Negligible shrinkage. Additives are also available to thicken if required, also to accelerate cure Obtainable 4D £58.05 per 2kg (Gel 10 and Gel 00); mouldlife.net  £37.58 per 1kg (500g each part), £50.54 per 2kg Kit combining 500g Platsil Gel 10+1kg Smiths Deadener £65.00 (Mouldlfe) Smith’s Deadener on it’s own £31.32 per kg (Oct2012).

The US firm SmoothOn also manufactures a range of silicones and additives for prosthetics with similar properties (i.e. Dragon Skin), available in the UK from Bentley Chemicals (see ‘Suppliers’) and 4D modelshop.

Further info
www.theeffectslab.com useful forum
http://www.polytek.com/cart/index.php?target=pages&page_id=platsil_gel_10_success_tips

prototype often used to refer to the original form or sculpted object from which first a mould and then a cast is made. This is my preferred way of referring to the original, rather than using the word ‘model’ or ‘master’

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