This is a companion to ‘quick view’ comparisons of casting materials in the section above. It makes use of the technical summaries I provide on my courses but I have added much more supporting information.
Example prices for the materials are from November 2019 and they are adjusted to include VAT. Full addresses for the suppliers can be found in the Suppliers section. Mixing ratios and properties are based on current use, but always check info supplied with product when bought in case of changes.
Silicone rubber is indeed related to silica,which means that it’s not so far removed from glass, oddly enough! Both share silica, commonly found as sand, as their basis ingredient. The standard, most-used silicone rubbers for home, hobby or small-business use are RTV silicones, which stands for ‘room temperature vulcanization’ and means that the rubber doesn’t need to be heated for curing. They are also likely to be condensation cure (otherwise known as ‘tin cure’) silicones which are characterised by mixing the rubber part with a very small amount, ranging from 2.5-10% by weight, of catalyst to enable it to cure. Addition cure (otherwise known as ‘platinum cure’) silicones are generally more specialised, cost more, and although perhaps easier to mix, two similar-looking parts 1:1, they are affected by a range of substances! The mixing/working time of RTV condensation cure silicones can vary greatly, from 5mins to 1 hour. Similarly curing time can be as little as 2 hours for some types, but a good 24 hours is standard.
Advantages excellent detail reproduction (long curing time is an advantage in this case because the silicone has the time to work its way into the finest details); hardly sticks to anything, except to itself (often no special barrier needed on prototype form, usually no release agent needed between mould and cast); versatile range (many types with differing properties) and adaptable (range of additives to modify them); available, many suppliers; high flexibility; durable (usually a large number of casts can be made and the mould will keep for years); strong structural memory (i.e. where a cured silicone mould is split, it will readily spring back to ‘heal’ the cut as it were, provided it is thick enough, and this is not much lost over time); low shrinkage; most standard silicones are heat-resistant up to about 200°C which means, for example, that polymer clays such as Super Sculpey can be baked inside them. There are other silicones which resist higher temperatures, suitable for white metal casting; easy cleanup of uncured silicone with white spirit
Not so good fairly costly at average £26 per kg (usually sold in kilos so, since silicones way on average 1.2 grams per ml, 1kg is 830 ml or cubic centimetres); needs fairly accurate dosing and thorough mixing; use of weighing scales strongly recommended; most often full 24hrs recommended wait for proper curing; addition cure silicones (also known as ‘Platinum cure’) if used, affected by presence of sulphur, some resins (see lexicon entry for list); not reusable once cured, but can be recycled by granulating old moulds and adding to new mix; unused silicone has a limited shelf life (manufacturers recommend on average 6 months, but in practice this could easily be doubled)
Lukasil 429 £22.64 per 1.1kg, £107.59 per 5.5kg (specialplasters.co.uk)Light pink colour. 10% catalyst by weight (12.8% by volume, so 100ml rubber needs 12.8ml catalyst). Shore A 20, red coloured catalyst, manufacturer’s recommended shelf life 6 months, total cure 24hrs (can be demoulded sooner). Add 0.5-1% thixo £6.60 per 100g (i.e. at 0.5% 100ml needs 0.5ml thixo). SG rubber 1.24, SG catalyst 1.00
Tiranti T28 £30.11 per 1kg, £121.82 per 5.25kg (Tiranti) Off white colour. 5% catalyst by weight (6.72% by volume, so 100ml rubber needs 6.72ml catalyst). Shore A 25, SG 1.29, withstands up to 200 centigrade, shelf life 6 months, pot-life 100mins (less with thixo), cure 24hrs. 0.5-2% thixo £7.44 per 50g (1% works well if thoroughly mixed). Can be coloured by adding up to 10% weight powder pigment. ‘T5 fast catalyst’ now available (coloured) to reduce cure to 2-2.5hrs (mixed 5%) but also reduces pot-life to c. 15mins. Costs extra £8.54 per 50g, £17.09 per 250g
Tiranti RTV101 £27.85 per1kg, £134.33 per5kg (Tiranti) Brick red,pourable. 3-5% catalyst by weight. Shore A 55, tough, designed for centrifugal casting using low-melt metals (withstands up to 316°C). Pot life c.1hr, cure 5-6hrs.
Tiranti T40 fast-cure £67.32 per 2kg (only size available). Light pink, sticky paste. Addition (Platinum) cure. 2 parts mixed 1:1, with just a few minutes working time and ready to be demoulded in 15-20mins. Shore A 40, can be thinned with silicone fluid. Suitable on skin for life-casting if barrier cream is used (avoid mouth and eyes).
ZA22 A ‘platinum cure’ aka ‘addition cure’ silicone with a very low viscosity; each part (white and blue) mixed 1:1 by weight; very fast curing. SG 1.1 Price a little higher than average at £27.50 for 1kg, £90 for 4kg at mbfg.co.uk
Addition cure silicones are easily inhibited e.g. by sulphur in some plasticine types, epoxy resin, polyester resin, shellac, superglue, condensation cure silicones etc.
Strictly speaking flexible polyurethane is not a ‘rubber’ it’s an ‘elastomer’ i.e. a flexible plastic .. but ‘rubber’ is a deal easier to say! Polyurethane rubbers are very like silicone rubbers in look and feel. I haven’t ‘featured’ any below because I’ve only used them a few times in the distant past. This is because using them is not as straightforward as silicone rubbers .. they have far too many ‘issues’ even though they can be cheaper. Generally silicone is well worth the extra cost unless you know exactly what you’re doing!
Advantages a higher Shore A polyurethane can sometimes be a better choice for heavy casting with abrasive materials such as concrete, often higher tear strength; useful if a ‘rubber’ cast is planned i.e. making a silicone mould then casting in polyurethane (no release agent needed with regular condensation cure silicones); very hard versions e.g Shore D 90 available; lower viscosity than silicone allows trapped air better means of escape
Not so good not advisable to cast polyurethane resin into a polyurethane rubber mould, will bond without special barrier agent and even then may be tricky; generally not as easily ‘releasing’ as silicone, will bond more forcibly to a variety of prototype materials unless special release agent is used; not as long-lasting as silicone; sometimes longer cure times and the need for heat-assisted curing; catalyst is a harmful isocyanate; proper curing is affected by moisture in the atmosphere, cannot be used directly on damp materials
Vinamold or Gelflex
These are the two (I think the only two) brand names for a form of re-meltable vinyl compound which comes in a solid, rubber-like block form. There is little difference between the two brands .. both have harder and softer versions, both melt at around the same temperature i.e. 140°C; both accept all casting materials, including silicone!
Advantages the reasons for using re-meltable vinyl are almost wholly to do with economy or speed. Inexpensive material at average £12 per kg .. and reusable! (used moulds can be cleaned, cut up and re-melted, many times as long as material isn’t ever overcooked); will take 30mins to 1hr to set, dependent on size of mould, and can be used straight away; can be applied to damp materials e.g. fresh clay
Not so good must be melted. i.e. special equipment and care needed; incomplete detail reproduction (it may cool before it can travel completely into deep detail); irregularities on mould surface (because it cools on contact with the prototype slight ‘tide lines’ may develop); prototype forms i.e. originals must be heat-resistant; general lack of versatility i.e. only two hardnesses, can only be melted and poured, cannot be applied in layers (if poured in layers, these are likely to separate); heat causing bubbles in mould (expanding air can sometimes bubble up from the prototype itself and affect the mould surface); limited mould life compared to silicone (casting with resins will wear moulds down very quickly!)
Health & safety re preparing special ‘kettles’ designed for the job are often advised (and probably mandatory in school environments) but these enclosed pots are expensive and I can vouch from doing it many times that it’s safe to heat the material up in a regular saucepan as long as certain important things are observed and constant attention paid. Melts at 150, constant watching/stirring needed
Tiranti’s Vinamold £13.10 per 1kg, £50.96 per 5kg (Tiranti) SG 1.00 (1kg Vinamold equals 1litre volume
Before flexible mouldmaking materials were discovered and became available, plaster was a standard mouldmaking material. Moulds often had to be made in many pieces, to part comfortably from original prototype or casts, because rigid plaster allows no undercutting at all. Plaster is still used for some forms of mouldmaking i.e. large-scale formers for fibreglass casting, or for ceramic slip-casting and for making rigid mould jackets
Advantages very cheap, even for good-quality brands; wide choice of types with differing properties; essential for ceramic absorption casting method (special, more porous type needed); excellent, quick material for mother moulds (mould jackets) and can be used in conjunction with jute scrim, or even glassfibre matting for extra strength
Not so good categorically no flexibility, so there can be no undercutting unless both prototype and casts are themselves flexible; only truly cheap if bought in standard amounts i.e. 25kg sacks and from proper specialist suppliers (as opposed to 1kg bags from Hobbycraft)
Crystacal R casting plaster £29.11 per 25kg (specialplasters.co.uk). Similar to above but even harder and stronger. Recommended mix for maximum strength is 2.86kg per litre water, but this mix does not pour well (2.2-2.5 makes more pourable mix). Pot life 10+mins, setting 15-20mins, demould 30+mins. Slightly longer working time makes this a good plaster for building up mould jackets using its intermediate ‘cream cheese’ state, but one has to work fast.
Pottery plaster £11.21 per 25kg (specialplasters.co.uk) A good, and very cheap plaster which is particularly porous, designed to be used to make moulds for absorption casting. This is a ‘beta’ plaster. Best mixed by eye, but manufacturer’s recommended optimum ratio is 1.5kg per litre water.
Mixing/working guide: the recommended ratio of plaster to water varies by type between 1:1 and 4:1 plaster to water by weight. Plaster is always distributed (sprinkled gently and evenly) into water, never the other way around. As a very rough average, plaster has a 10min pot-life and will take 30mins to set, during which time it will become warm. It is best to wait until the set plaster has become cool to the touch before it is unpacked. As an equally rough average for mixing a specific amount of plaster; the amount of water measured out will roughly double in volume when the plaster is added and mixed i.e. 100ml of water will yield 200ml of plaster mix. This is based on experience of mixing relatively dense casting plasters (around 2.5:1 plaster to water ratio) so it’s likely that the cheaper, less dense plasters (around 1.5:1 plaster to water ratio) will yield a little less than double.
Alginate is commonly used as a short-lived mouldmaking material for taking impressions from a live model during life-casting. It was developed for dentistry, to enable quick and accurate impressions to be taken in patients’ mouths. I’ve always introduced alginate by saying it is derived from algae, i.e. seaweed! In a sense this is true, but what comes from the seaweed constitutes only a small fraction of the powder sold as ‘alginate’ for life-casting. What is extracted from (principally brown) seaweed is alginic acid which is then further refined and converted to sodium alginate, a gum, and it is this gum which serves as a flexible binding agent for all the other ingredients which are used to make the final ‘alginate’ powder. This powder has to be mixed quickly with water to make a consistent paste and applied just as quickly to the live model. The ideal paste should have an even, creamy consistency but not drip from the mixing stick. Different brands tend to have different colours (colouration added); white, light blue or light green.
Advantages skin-friendly; excellent detail reproduction; very fast setting (ranging from 2mins ‘normal set’ to 4mins ‘slow set’ type); fairly easy to mix (1:1 by rough volume with water for standard life-casting applications); fairly inexpensive (average price £10 for 500g, dependent on how it’s used can be up to a third of the price of silicone); there are some versions which are ‘engineered’ to change colour on setting (this was developed for dentistry, to aid quick identification of mould readiness if hard to see or feel, and is hardly necessary when used for life-casting); the temperature of the water used has an effect .. warm water will speed up setting, cold water will slow it down.
Not so good very low tear strength (only a little tougher than tofu ..!); mould does not keep (should be cast into soon after making, at least on the same day. It will dry out, contracting and distorting as it does so); needs supporting (in life-casting this is often done by applying a plaster-bandage shell over it); normally only suitable for casting with plaster or wax, but see below; some practise needed in applying it quickly and properly; alginate compounds are sensitive to hard water (the extra calcium and magnesium in hard water will affect the chemical balance and the material may set lumpy or in extreme cases not set at all. If your water is hard .. you will have to use distilled water)
Some brands, such as the one featured below, are suitable for mixing with double the amount of water ( so 100ml alginate to 200ml water ) to create a pourable liquid which will set. I’ve made conventional 1-piece moulds for flat objects this way .. but very carefully! I’ve even cast into them with polyurethane resin! Once set and detached from the prototype (the original object) the mould surface needs to be delicately dried with tissues and then greased with Vaseline. See Casting polyurethane resin into alginate in the mouldmaking section of ‘Materials’.
Flints Slow-set Alginate £10.18 per 550g (flints.co.uk) Pale green. Slow-set type gives 4mins for mixing then 2mins before setting. Mix 1:1 powder with water by volume for brushable paste, or use twice the amount of water to make a pourable mix. At 1:2 ratio 100ml (settled) of powder will yield 200ml pourable mix.
Available also from Tiranti £11.33 per 500g but the directions only mention making a paste, not a liquid version.
Latex is a natural liquid rubber which sets by evaporation of water content. It is still used sometimes for very basic mouldmaking, i.e. the ‘sock’ method .. applying successive coats to a simple form, peeling off as a complete skin and filling this with plaster
Advantages inexpensive; ready to use, no mixing; no health&safety issues; surprisingly strong, good tear strength; fairly good detail reproduction
Not so good 10% shrinkage; not compatible with Vaseline; needs to be applied in thin layers, each drying before the next is applied; can be easily coloured in-the-mix with water-based paints i.e. acrylic, tempera, but not so easily painted
£12.34 per kg (Tiranti)
Excellent article with many useful points! Thanks. Saved me a ton of work!
Hi, I want to make a fired clay copy of an ornate picture frame. I have looked at various sites which imply that natural clay can’t be used with some mould-making materials like liquid silicone rubber. Is there a material I could take a mould off the picture frame with and then pour clay slip into? I have tried using expensive putty-like silicone rubber (mixing two parts together)and making a mould of one corner of the frame, filling that with plaster of paris, coating the plaster with soft soap and then pouring on another layer of plaster of Paris which was fiddly and not very satisfactory. The picture frame has several undercuts so the mould needs to be flexible. I have read your excellent article about mould making!
No, I’m sorry to say there’s no mould making material I know of which you can cast clay into (whether slip or press-moulding) and expect it to turn firm enough to take out intact .. except absorbent plaster, that is! All I can suggest is that you make a regular flexible silicone mould and cast in ‘Terracotta Cast’ (Tiranti), a special form of plaster, which simulates the look of fired clay.
Thanks very much, that’s very helpful.
I have some old latex molds. If I cast polyurethane resin into them, what do I use as a mold release agent?
A good one! It’s usually advised that latex doesn’t like Vaseline i.e. it can’t set properly on it .. and I’ve always followed that .. but I don’t know whether that applies if just greasing ‘cured’ latex as an extra release from resin. Maybe it does .. but no harm in testing? Otherwise you could possibly use a common oil i.e. baby oil, cooking oil, rape seed oil?
Can you recommend a starter kit for an absolute beginner please?
I can actually! In the first place I have a fairly thorough list of the things one needs (maybe not all at once, or for everything) in my article ‘Beginner’s Basics ..’ in ‘Methods/mouldmaking and casting’. You might have seen it? I’ve also just seen that mbfg.co.uk do ‘mouldmaking and casting kits’ i.e. starting with £27.50 (half kilo silicone + 1kilo polyurethane resin + mixing beakers, latex gloves, mixing sticks). I can’t personally vouch for the Polycraft brand materials (have yet to use them) but I’ve looked at the technical info and they’re practically identical to what I use/recommend.