The PRINCIPLES and PRACTICE of MAKING
SILICONE MOLDS and RESIN CASTING

Welcome to the 21st centuries equivalent of the ancient art of Alchemy. Starting with basic sculpting skills, mixing lotions and potions of silicones & pouring various mixes of resins or plasters, adding pigments, precious metals and ageing patinas, you can create new and exciting artefacts of the future.

I hope you find the following interesting, more than useful, not too challenging, and inspirational even. This is one of life’s journeys that you start out with a small step and may quickly and easily end up taking giant leaps, as I like to think I did, some years ago.

The Paradoxx philosophy for making molds and casting :

Start at the end, nothing hard to understand so far! By this I mean start with the piece you plan to finish up with clearly lodged in your minds eye. Knowing what you want at the end makes choosing the correct raw materials, silicones, resins, plasters, pigments, paints etc, much simpler.

If you can present you finished concept clearly to your raw materials supplier, it will make life much easier all round and help ensure you purchase the right products.

Silicones: an introduction

Most silicones are suitable for multiple castings, a few “namely alginates” are only good for “one shot”. Some silicones are “skin safe”, some are “food safe”

Others are used for casting white metals (today’s version of lead ) most are used by pouring into a mold box, over an original piece that’s to be replicated and some can be brushed over the surface of the original work.

Mixing ratios (all RJV2 silicones are ‘two part’ products) vary from a 50/50 mix, to 100:2 parts catalyst. Some are measured/mixed by weight others by volume. It is always easier to measure by weight than volume, when the catalyst or 2nd half of the silicone is to be added at the rate of 2%, you need accurate scales, if you add 3% you are in fact out by 50% and the likely outcome is the silicone will set while you’re pouring it, a very expensive drama!

Silicones have different work or ‘gel’ times, they set at different speeds and in varying degrees of hardness & flexibility. In basic terms there are 2 types of silicones those that are tin based or ‘condensation’ cured and the others that are platinum based or ‘addition’ cured, generally the two types are not compatible.

“How many pieces can I cast?” Production from a silicone mold will vary according to what you’re pouring into it and if there are severe undercuts. If you were casting Jose (who you met a little later) in resin, you might well expect to get 40 or 50 casts from a mold.

With so many options, 15 plus types of silicone and even more resins on the suppliers shelf, your best option is to know what you want to do, explain it clearly and let the supplier recommend the right product for your application.

Handling and mixing of silicones and resins

When ever you purchase raw material you’ve not used before, ask for a Safety Data Sheet, they are freely available, the supplier will not think the less of you because you ask. Then having asked and received, you need to do something un-Australian! READ THEM and BELIEVE WHAT YOU READ. Safety sheets exist for good reason, remember you are messing with chemicals and there are chemicals out there that don’t like you! a little harsh perhaps as they don’t know the real you, but it’s true. Some folks are allergic to polyurethane resins, usually manifest as a skin rash, which will miraculously go away if you take the resin away. Many schools and colleges have banned epoxy resins, these products truly deserve your respect, read the HAZARDS and believe them. Safety and data sheets advise ‘hazards, flammability, storage, handling’ should you use gloves, mask, googles, what to do if spilt, swallowed etc. Also mixing ratios, work and gel times, etc. When safety sheets talk about ventilation, they don’t mean opening the back door 1/2 way. If a mask is recommended, use a real mask, not a paper one! Never forget, you are messing with chemicals so follow the info and measure accurately.

Be aware that resins and silicones may be mixed / measured by weight and others by volume, read the data. Calibrated measuring cups and / or accurate scales are the order of the day. Don’t fall into the trap of using a cup, marking it in the middle and saying that’s 50 50, It’s not likely to be correct, the top of the half of the cup will hold more than the bottom half, ask any self respecting beer drinker.

James Bond may well like his martini dry and “shaken, not stirred” but silicones and resins are a tad more fussy, than Mr Bond and insist on being extremely well stirred. Don’t try to mix using a skewer or a fork, that’s not going to do it, lash out, buy some tongue depressors and get serious.

Until you get used to the mixing process, clear plastic cups are the best option, you can see what’s going on. Mix materials well & quickly, don’t forget to scrape the walls& base of the cup. If raw materials are not mixed correctly they will just sit in the mold and look at you as if to say “well what do you expect me to do if you don’t do your bit properly?”. If the temperature in your work area is outside recommended limits and you go ahead, expect the unexpected!

Silicone is made from Silica as is glass, silicon may bond to glass & to glazed ceramics. If you’re planning to apply silicone to a ceramic piece, do a small test first and don’t do your test on the underside of the piece, even though it may be glazed the glaze will inevitably be much lighter than on the decorative surface. Beyond that silicone doesn’t really bond to anything, other than itself that is and of course your best jumper or new jeans. You can bond wet silicones (of the same type) to dry or add wet to wet.

Most polyurethane resins get quite hot while setting, it’s an exothermic process, this applies to fast setting resins, ie those that will de mould within an hour. They have short work / gel times, usually in the 2 – 3 minutes, no time for messing about, if you get half way thru a pour and the resin starts setting in the cup you waited too long. If the phone rings in the middle of a pour, let it, they will call back. Best guide here, hold the cup in the palm of your hand, mix like there’s no tomorrow (perhaps there isn’t!) as soon as you feel the slightest change in temperature, it’s time to pour. Resin or silicone splashed on your skin is a brief nuisance, wipe it off after you’ve finished pouring.

Silicones on the other hand don’t get hot during the setting process. To help ensure silicones are well mixed, most ( but not all) have one part a distinctively different colour to the other. So you can see if you’ve done a good job and deserve a chocolate bickie. Where the catalyst is clear and if you’re not confident of your mixing skills, you can add a colouring agent to one part.

In both materials work times, gel times & setting times vary, read the data sheets.

The Open or Single Part Mold:

An open mold is almost the simplest mold you can make and the right place for us to begin. This kind of mold can be used for any casting that will be flat on the back, like a plaque or broach. That said given the flex of most silicones it can still be a complicated piece with numerous undercuts.

Creating an open mold:

Lay the original piece you plan to replicate on a flat work surface, fix it in place with a dab of superglue, if the original is light weight it’s likely to float to the top of the silicon.

I use an oil based clay, which doesn’t dry out and shrink & can be re-used time

and time again, seal all the way round the edges of the piece to prevent silicon from flowing underneath. Thick and creamy as they are, silicones will find their way into the thinnest gaps.

Build a mold box around the piece, the box needs to be at least a centimetre higher than the highest point on your piece, and should be 1 to 2 centimetres wider all round. A mold box can be built from almost any material, oil based clay, aluminium, plastic cardboard, lego blocks, timber. Seal the corners of the box and the edges with clay. Make sure the box is held together firmly enough so that the weight of wet silicone wont cause it to open.


Fixed into the bed of clay, edges dry connected and 3 sides of the box

Mark at least one wall of the box at a height 1-2 cm above the highest point of the original piece. Mix the silicone and pour, slow but steady into an open area of the box, resist the temptation to pour it all over the piece, or where ever the level appears to be low, silicones will self level quickly enough, fill the box to your mark.

Small air bubbles will form on the surface of most silicones, ignore them, if you pour as directed the chances of a bubble on the important design surface are 10 zillion to 46.

Go have a cup of tea and leave the silicon to do its thing.

Setting times under ideal conditions vary from 15 mins to 24 hours, depending on the type of silicone and such factors as humidity and room temperature.

When the silicon is set, pull down the damn walls and prise it gently up off the work surface, lay it on its back and lift the original piece out.

You now have a negative, 3D mold of your original, with the right material poured into this mold, you will create an exact replica of your original, ready to paint, patina or what ever you choose. How easy was that!


Finished open mold


The new replica, the base was also cast in an open mold, it’s fixed to the head using a self tapping screw.

Silicone mold design:

Design is the most important part of every “2 part” mold. A poorly designed mold may result in awkward join lines running through the middle of the face of your Venus De Milo, where it may be difficult, if not impossible to clean properly. Poorly designed molds may distort castings, undercuts may cause air bubbles in the casting process, under chins or on the tips of delicately upturned noses. Don’t forget to consider wether or not the new casting can be easily removed from the mold without putting too much stress on it.

If you’re planning a cold cast metal finish, poor mold design can prevent laying down good metal skins. Brushing gel coats, used in cold casting, do not run like other resins do and important details can be lost, molds for this application need to be as open as possible. Often as not the mould for a figurative piece can be built in two parts, whereas a plaster mold for the same piece in ceramic, might come in at 10 or 12 pieces. NO! Sadly, ceramic slip will not set in silicone.

Two Part silicone molds :

This is where the work really begins

Using a two part mold we can replicate 3 dimensional objects such as sculptures. These pieces can be as complicated, as a ballerina, standing on one foot, arms up and outstretched or a jumping horse. For this workshop we will use a small bust of the internationally famous Tin Whistler Jose.

Remember join lines don’t need to divide the mold in neat halves, fact is it’s most unlikely to be straight, flat or level. It shouldn’t cut across Jose’s cute face. Most pieces have a front and back, if you were going to sit Jose on your mantle piece, you would obviously have his delicate features beaming out across the room, so join lines down his neck and across his head should be set to the back to make them less visible.

So to work : Centre a lump of clay on your work surface, and as our join line is to the back of his head, push Jose face down into the clay, ensure he is located firmly and that the base is as close to vertical to the work surface as your eye will allow.

Roll small lumps of clay into logs and place them around Jose’s outline, always using small logs, build this “blocking in” up to within 3 – 4 millimetres of the join line. Don’t pack these logs down hard on each other. Walls should be approx 1.5 cm in width all round, remember it need not be level nor smooth, in fact better if it’s not. So long as your structure is sound there’s no need to fill all the area beneath Jose.

The last level, make longer logs this time and flatten them between your fingers so that they are 2 – 3 mm thick and 10 – 15mm wide. Lay these last flat logs down and press them firmly against Jose, but don’t force it. Complete this last layer all the way round.

Now for the smoothing and dry connection process, use a clay tool, not your fingers, and run the logs of clay lightly together.


And this is where the important work really begins!

We need to make a “dry connection” between Jose’s head and the clay, what’s meant by a ‘dry connection’. Remember wet silicone will escape thru the thinnest space, it’s not inexpensive so we don’t want to waste any, and if it escapes into the wrong place all your hard done work can be ruined. So first we need to firmly but gently push the clay against Jose’s head following the join line round as we go.

Now this is where I get to be a little demanding, not only do I want a dry connection, but I want a very, very tidy connection.

The point where the clay and Jose meet needs to be ‘flat’, this small connection point and the first 2 – 3 mm’s of clay moving away from Jose needs to more or less at 90% to the surface of the original. What’s required in the final molds is that the two edges of the mold sit together firm and flat, skinny floppy edges can fold when the mold is joined, the result will be an untidy cast, leaked resin or both.


A good flat, dry connection


Nope not good enough!

Don’t push all the clay home then run the wood tool around pushing against the piece, this will create a bow wave effect and a thin flap, exactly what we don’t want. Place the edge of the tool against Jose and ride down pushing the clay flat, move the tool a mm and do it again, and again and work all the way round. If you follow the join line as per the pic, you will find that the surface is far from even as it runs from his head to the top of the ears, around the middle of his ears then to the back of the neck.

When you think you’ve finished!, turn your work surface around into a new light, tip it and check to see its as good as you first thought it was and that it’s dry.

Now a damn wall is built around the clay and you need to mark one internal wall with the height to pour the silicon too, at least 10mm over the highest point of the back of Jose’s head.

What’s left? You need to fill in the gaps between the clay work already done and the damn walls. Do this slowly and carefully so as not to dislodge the finished work, drop in loose bits of clay and pack them down a little, when you are at the same level as the finished clay, lightly run the surfaces together, and make this join & the join to the mold box walls ‘dry’. The 4th wall is built hard up against the base of the piece, when the mold is complete, this will create the pour hole.

Almost there, some location holes, created using the blunt end of a pencil, drill it 2 or 3 mm into the clay, no more, and the hard work is done.

Estimate the volume of silicon required, approx 1 gram for every cubic cm of empty space in the mold box up to the pour line. Mix according to manufacturers directions and pour, remembering to pour into an empty corner.

Second half of the Two Part mold process:

With the first pour of silicone set, it’s time for the easy part of the process. Gently pull down the damn walls, now you have a small block of silicone sitting on a block of clay, with open palms hold opposite sides of the block and rock it until it comes free of the work surface.

Lift everything as one, turn it over and set it back down again, clay on top.

Now piece by piece, log by log deconstruct your clay work, don’t rip at it, for once in your life be gentle! Note how the logs have taken a good grip on each other and tend to want to all come away in large chunks, it’s not what we want and we are in charge. If the clay comes up in large chunks it may dislodge Jose from the silicone, this we definitely do not want. If he is dislodged, he simply will not go back ‘dry’ and then the risk of the 2nd silicone pour ruining the first is high, now you see the reason for building in small logs.

Once all the clay is removed you can clean up the little bits especially round the connection between Jose and the silicone, don’t worry too much about small spreads of clay on the silicone surface.

Put the silicone block in the middle of your workspace and rebuild the damn walls, fix them in place, seal up the corners and mark the new level you need to pour to. Mix the 2nd lot of silicone and pour it over the 1st half of the mold, and in the process ruin all our hard work in seconds, sheeeeesh!

Were you paying attention earlier? Remember wet silicone (of the same type) will bond to dry silicone, so if we just pour one on top of the other we will end up with a block of silicon and Jose trapped forever in the middle. To prevent this international catastrophe we apply a layer of release agent to the dry silicone surface and then pour the second layer of silicone. I like to apply release agent to the sides of the first silicone block as well as the surface.

“Which release agent?” I hear you cry out in the wilderness, and verily I say unto you “go back to paragraph one” where I refer you to your material supplier. Vaseline can be used as a release agent, but the trick here is to be 101% certain you have completely covered the surface and that you haven’t built up lumps that may distort the mould, A better option are the range of appropriate, spray on release agents.

When the second pour is set, tear down the damn damn walls with gay abandon or anyone else who wants to assist. Trim any untidy edges with a very sharp blade and turn the block of silicon so you can see the base, find the join line and pull the two halves of the mold apart. You might need to pull hard to the point you think your going to tear the silicon, fear not it won’t happen, once you get started it will open easily. Set Jose free and there embossed forever in the silicon is his nemesis Negative Tin Whistler Jose in all his glory.

Pouring your replica:

First rule: be patient, yeah right! Yes you can cast into a mould that’s 2 minutes old, but to get the best service from your mould leave it to cure properly, times vary, read the data sheets!!

When it’s finally set and removed from the mold you should have a prefect replica of the original piece. It’s you discover some imperfection in the replica and ponder from whence it cometh! Ninety nine times out of 100 the answer will be that this imperfection is in the original, its just that you didn’t notice it before. So it’s a good idea to check originals out at the beginning of the process, not the end.

Phinishing Painting Pigmenting and Patinas

Despite their raw liquid colours, most polyurethanes will set either white or creamy beige.

Even though it will be rigid when it comes out of the mold, assuming you were patient enough to let the piece cool, resins can take several days before they are fully set and stop giving off fumes which you wont be ware of and needn’t worry about. But these fumes can make painting difficult, so put the piece aside for a few days and go do other stuff. When it comes time to paint, there are as many paint options as there are painting theories. Most resin painters would suggest the first step is to apply a coat of Metal Primer, these can be purchased in tins or spray packs, spray packs give more control. Apply a thin coat all over, don’t try to achieve full coverage with the first coat that will put too much paint on. Let the first coat dry, then apply a second coat. Now you have a base that your top coats will adhere to readily. Apart from the big hardware gigantums, the local model / hobby shop, should have all your painting needs and good advice. Advice is best delivered by someone who actually uses the product rather than someone who just sells stuff

Pigments will do some of the hard work for you. There is a large range of pigments that can be added to the resin prior to pouring to give you a solid coloured replica, straight from the mold. Measure the first component of your resin into the mixing cup, then add pigment and mix it thoroughly, that done add the second part of the resin, this way you don’t loose working time. If you add too much pigment you will mess with the chemical balance of the resin. Remember the natural color of the resin is white or off white and this will tend to soften the finished colour. White setting resin, wet in the mixing cup with red pigment may look sexy lipstick red, when it sets it’s more likely to be closer to pretty bunny pink. Adding too much pigment, especially cream types may mess with the chemical balance of the resin, so experiment. Clear polyurethane resins are also available along with a range of transparent pigments or tints.

Metal paints, as their name implies are paints containing pure metals, bronze, brass, etc in a paint solution. You will need to go thru the same preparation using primer and then you can brush the metal paints over the top. Metal paints are great when combined with ageing Patinas, or coloured waxes.

Patinas come in a wide range of colours or effects, from rust to blue bronze, with about 2 million options in-between. Metal paints work on almost any surface including plasters, a well finished plaster sculpture can be most deceptive to the eye. Patinas are fun, if your metal paint is wet and you apply a patina the piece will age before your eyes. Don’t get too close, most of them have a pungent smell. The best method of application is a small air pressure pack kit, alternatively Patinas can be applied with a padded cloth or paper. Patinas will continue to change on a metal painted surface until you choose to seal it, with the likes of shellac.

If you want to age something you have coated with metal paint and your not in a hurry, put it out in the weather and let mother nature do it for you, making sure your piece is weather proof.

Resins tend to fade rapidly in direct sunlight and thought should be given to placement of finished pieces or look at UV additives.

Waxes are quick and easy to apply, literally wipe on, wipe off, working best on textured surfaces. You need to be quick, it’s basically a ‘ wipe on wipe off ‘ application.

When you are confident about mixing the resins quickly and efficiently try this. Open and stir your pigment, mix both parts of the resin together until they are ready to pour, flick several drops of pigment into the resin, a quick swirl, don’t mix it and pour.

Alchemy is always about experimenting, sometimes a mistake in the process can bring interesting results, experiment on a small scale, there’s no need to pour 1 kilo of resin into a mold to see what happens, tip it on one side and fill a small pocket.

Here endeth the lesson,
just as well I can see my bus coming.