Chapter 8 - Silicone Caulk Molds

In this Chapter we will discuss using Silicone Caulk as a material for mold making. It is important to realize that caulking comes in 31+ flavors and it is important to select the correct stuff if you want to use it for making molds. I use 100% clear silicone rubber sealant. It is available at most any building supply store, farm supply store, Wal-Mart and any other store that has a 'Hardware' section in it.

Many will argue that Silicone is not the best material to make molds with. And I agree! But for the cost it is worth considering and using. Silicone caulk has pros and cons just like any other material used to make molds. These are listed below.


  • Availability - This stuff is literally in about any large store you can walk in to.
  • Cost - The caulk generally sells from $2.50 - $3.50 (USD) making it usable on a shoe string budget.
  • It dries flexible making it usable for a number of mold types.


  • You have to layer your mold. The thicker you layers the longer it takes to dry between layers and you cannot go much thicker than .25 inches or 0.5 cm.
  • It puts off an odor that smells like vinegar (Acetic Acid).
  • It has to be thinned down for the first coats to work well.
  • It is difficult to spread without leaving spikes and burrs on the outside in its unthinned state.

*** UPDATE *** - Thursday 25 January 2007

Mr. Gary Boyd has done some research on the use of silicone caulk as a mold making agent. He has Kindly allowed me to post his information here to share with you. Gary done his homework on this, you will find a list of reference at the end. Thanks Gary for helping us at the Heap out!

This is some work I did a while ago, might be helpful. A little different results, probably due to proportions used.

Inexpensive mold making using single part silicone sealant The straight dope for casters.

Garry Boyd

This work is the results of some research into modifying cheap silicone RTV in order to create high resolution silicone molds from a single part silicone.

Caution: Your health and safety is your concern. While all products discussed in this article are readily available you should check the MSDS yourself before proceeding. A method of fume extraction is recommended. Mixing chemicals may create unknown reactions which could be hazardous.

Chemistry: Most of these sealants consist of two fairly complex chemicals. However the actual curing reaction is pretty simple. In simple terms, acetic acid is exchanged with moisture in order to set the product. The silicone forms a skin in reaction to humidity in the air, and further acetic acid leaches through an increasingly thicker skin over the life of the sealant. Eventually the silicone becomes hard and no longer seals. This can take many years.

When used for mold making these products have a number of undesirable properties straight from the tube. They are too thick, causing loss of resolution. If applied too thick you may have uncured spots which spoil the mold. Silicone sticks to silicone making two part molds difficult.

There are 2 main approaches to these problems which have been used. These are using water based methods to make the setting time quicker, and the product less sticky. The other method uses solvents to make the product more runny. What follows is the results of my tests of these methods and observations.

Water methods: 2 main methods were investigated: Adding humectants such as glycerine, and flooding with water. A blend of silicone, glycerine and sometimes a little acrylic paint is used. I also experimented with Mr Bubbles bubble mix which contains glycerine, water and soap. These mixtures set fast and become extremely grainy in texture. The paint speeds things up considerably. They do not get good detail as they are so porous. They are not sticky at all and leach slime for days or weeks and eventually become crumbly. The second wet method involves squirting silicone into a can of water and massaging it until most of the acetic acid is neutralised. You then have a ball of non sticky silicone which can be pressed onto the object to form a mold. This method is difficult to get right due the initial stickiness in the water. It sets fast but is hard to work and does not retain good surface detail.

Solvent methods: A number of chemicals are mentioned such as naphtha (lighter fluid) xylene etc. I have also experimented with a number of other readily available solvents. My best results have come from using mineral turpentine. Naphtha in the form of zippo lighter fluid gave similar results. It sets a little quicker, but is a bit cloudier. CRC also works, but is cloudier still. Turpentine is cheap and good. Thoroughly blend to the runniness you need. About 50/50 is a good starting point. The cure time is very much extended by thinning, up to a couple of days. The more you thin, the longer the cure. These mixtures are easily poured, but remain extremely sticky. You need a release agent on your model.

Release agents: I have tried a variety of releases as follows:

  • Graphite powder: Is absorbed into the silicone. Hopeless.
  • Cooking oil: Pretty effective, use spray on types, make sure you get a good brand as some of them are very blobby.
  • Petroleum jelly: vaseline and rexwax both work, but are too thick for good detail, need thinning. Played around with thinning and spraying with an airbrush, but cooking oil spray is much easier.
  • Glycerine: This is quite an interesting release as it will form a rapid skin where the silicone touches. Worth more experimentation.

De airing: No de airing of the silicone has been done in these early experiments. For the best possible surface resolution, the silicone should be de aired under vacuum before pouring. After pouring, pressure or vacuum can be used to improve surface detail.

Mold support and sprue materials: Oil clay (plasticene) often contains sulphur products, especially in the cheap brands. This will react with the silicone forming a gooey sludgy useless mess. If you use plasticene to support your model or form sprues, it must be thoroughly sealed with varnish, emulsion or laminating spray. Water based clay will easily release from silicone. However the water based releases will affect the surface.

Rapid prototyping method:

This is a preliminary method to create a quick two part mold without needing to build up a bed of clay to support the model. Not fully tested yet.

Form a mold box from a porous material such as cardboard. Use hotmelt glue to join the sides. Coat the inside with a thin coat of glycerine. This is to form a quick skin on the outer layer of silicone. Thin enough silicone with turpentine to a runny consistency to fill the mold. Add about 5% Mr Bubbles or a couple of drops of glycerine in water to half of the silicone mix. Mix thoroughly. This will ensure the acetic acid reacts throughout the mix with no uncured spots.

Pour half the mix into the mold box and leave for half hour or so. Coat model and sprue with parting agent and place into the mix and push in to parting line. The best parting agent for this method is glycerine, although spray on oil will also work. Coat a couple of marbles and place them as locators for the finished mold. The silicone should be reasonably firm and jelly like, not too runny at this stage. If it is too runny your model will sink below the parting line. Lightly mist the surface with glycerine/water mix, or Mr Bubbles. A skin should form in 30-120 mins. Pull out the marbles and mist the holes left with parting agent. The holes may close up a little, but if the silicone is still runny, it is too early. Leave until a skin forms. It should be springy to the touch, not sticky at all. Mist again with parting agent. Add 5% water/glycerine mix to the second half of the silicone. Pour in and leave at least twelve hours to set up. Pull away the mold box and you should be able to open the mold and remove the model. Too slow? Set the top skin and then place the whole mold in a bucket of water for an hour or two.


This work is purely bush science experiments based around anecdotal evidence from internet discussions. Many of the methods put forward just plain do not work, I doubt the authors have tried them. Some of them work but in such a poor way that they are not worth the effort. I have devised these methods by attempting to understand the chemistry and carrying out actual experiments to the point that I am confident I can produce a useable high definition mold. Your results may vary. These molds will never be as good as 2 part silicone molds. They will not last as long. However, they are about 1/20th the cost and use off the shelf items. My tests used Ados glass and window sealant, which is acid cure, ie it is corrosive to certain metals. Later tests used Ados roof and gutter sealant which is neutral cure. There is a discernable difference between the reactions of these two products, with the neutral cure product reacting more slowly to various modifiers. . .

Parting agents: According to the literature, it is difficult to prevent silicone sticking to itself. However, I have been able to use both pure glycerine and canola oil spray to separate layers of silicone successfully. There is a commercial silicone release agent available in a spray can. Its ingredients are: a propellent, dichlorotetrafluoro-ethane, which is a freezing agent, and a polymeric resin/wax blend. You could probably replicate this with a can of spray freeze as sold in electronic stores, and a silicone wax furniture polish. I have carried out some experiments using butane lighter gas and observed that chilling the surface forms an instant tough skin. I assume this product works by skinning the surface and then forming a wax barrier.

The ideal recipe requires enough solvent to flow freely into surface details. The thinned silicone is smooth and syrupy with no lumps. Water or wetting agents are counter productive for definition as they make the silicone lumpy and hard. However they prevent the silicone sticking and shorten the cure time. Finding the ideal balance for your purposes will require some experimentation. Manipulating the temperature and humidity would probably be worthwhile.

Chemistry: Single part silicones only begin to cure when a reaction occurs with moisture in the air. When a solvent is added it decreases the viscosity by diluting the silicone. However it also seems a difference in the chemical composition takes place. It becomes clearer, more sticky and cure time is greatly extended. Most likely the acetic acid is no longer able to react with the moisture in the same way. While the volume does shrink a little, it is still a greater mass than the silicone alone after curing, therefore the solvent must still be bound in the mass. A better solvent may exist. Various petroleum distillates could be tested. Organosilicon solvents may be more closely linked to the silicone chemical structure. These are commonly used in dry cleaning, however they are far more toxic, hard to find and expensive than petroleum based solvents.

The water acts as a catalyst. Glycerine has a high affinity for water. Both glycerine and detergent are used to lower the surface tension of water, allowing smaller droplets to form. In order to thoroughly disperse the water and allow it to fully react with the acid and cure the silicone we need the smallest possible droplet size. This will decrease the lumpiness in the cured product. There may be other commonly available surfactants that are more compatible with silicone. A likely place to look would be hair shampoos, cutting oils, or liquor stores.

Traditional methods: The most common way of using silicone sealants is straight from the tube, smoothing on with a wet finger and building up layers over time. This results in a very uneven curing during to placing wet silicone over a hardened skin layer. Many artists use a saliva wetted finger. This is probably due to the lower surface tension. While silicone is low toxicity, it is certainly not designed for human consumption.

These moulds become brittle rapidly. Even moderate thinning to a thick brushable consistency for the first coat would be better, misting over with water to form an even surface skin. Avoiding wetting and storing in a moisture free environment would lengthen the life of these molds by slowing down the catalysing of acetic acid and keeping them more pliable.


*** UPDATE *** - Thursday 25 January 2007

A message was posted to the Casting Hobby forum from a fellow that I thought would also be helpful. I do not know how to reach the original author but here is a link to his original posting. I am placing it here as a convenience to you. If however, I am contacted by the original author and they wish to have it removed, I will have to comply.

Using Silicone Caulk as a Mold Material

I was reading the back posts and saw that some of you have been using silicone caulk for making molds. I have been working with a variety of silicones for some time and thought I'd post some information that might be helpful.

There are three basic types of silicone. The first two are two-part silicones which must be measured and mixed just prior to use.

  1. Platinum cure (addition) silicones are extremely pure and can produce extremely accurate molds that last indefinately. They are used for medical and theatrical prosthetics, medical equipment, cookware, candy molds, and baby bottle nipples. Unfortunately, these are also the touchiest of the silicones to work with and their cure can be hindered by moisture, sulfur, latex, tin, loud cursing and bad hairstyles.
  2. Tin cure (condensation) silicones are commonly used as a mold making material for art and industry. They are not approved for long term contact with the skin, or for cookware or other food contact, though some are used in making seals for potable water supplies. It is also long lasting and makes accurate molds with a life of several years. They are still very stable and safe compounds for a wide variety of applications. They will cure underwater and actually require some moisture in their chemical process. In fact, there is very little that will inhibit the cure of a condensation silicone.
  3. The remaining class is one-part, self curing silicones like caulking and aquarium sealant. They are a variant of tin cure and are sold in air-tight tubes. These are further divided into two subclasses based on their catalyst:
    1.  Acetoxy - these are the typical ones you will find at your home centers etc. They have a strong vinegar (acetic acid) odor while curing.
    2. Oxime - are referred to as odorless cure silicones and can be found in some building supply stores but are usually more expensive.

The biggest problems with using silicone caulking for molds is that it is rather thick and easily traps air, and that it will not cure properly in very thick applications. These can both be remedied in the same way.

The reason they won't cure in thick layers is that they require the moisture in air to cure. Thick layers develop an air-tight skin, effectively resealing the caulking underneath. In applications more than 1/4 inch thick, you can often come back days later and the underside will still be soft (if it is against a non-water bearing material like plasticine clay) or will cure from both sides and have a pocket of goo in the middle.

The way to solve this problem is to get some moisture throughout the silicone. You can't just mix in water, it won't mix well and you'll end up with a mess. The two products I have found that work best are glycerine (available at any pharmacy or in the health and beauty department of most chain stores) and acrylic (not oil) artists or craft paint.

Start with clear 100% silicone caulk, like GE I or GE II, or DAP 100% Silicone Caulk. Squeeze out the amount you will need into a plastic cup that is large enough to give you stirring room. For each ounce of caulk, add four or five drops of glycerine and a drop of acrylic paint. Use a wooden craft stick to stir until you get a uniform color trying to avoid trapping any more air than necessary.

You can use just the acrylic paint, but I like adding the glycerine because it improves the cure and also reduces the adhesive qualities of the silicone, making it easier to remove from the model. You could also do this with just the glycerine, but it is more difficult to tell when you have obtained a uniform blend.

Don't overdo it with either the acrylic or the glycerine as more than a few drops per ounce will result in a weaker end product. You cannot thin caulking with these materials without sacrificing a lot of the good characteristics of the silicone.

Once you get a uniform color, you have from 15 minutes to an hour before the product begins to thicken, depending on the temperature and humidity. Working in a cool dry environment will extend your application window. If you are outdoors in southern Florida, in August, work fast.

Adequate cure for handling should take under two hours and it will cure evenly throughout, rather than from the surface inward. Again, heat and humidity will speed things up.

I like to brush on a thin layer first, getting out all the air bubbles and making sure you have good contact everywhere. Then a thicker coat can be spatulated on. Usually about 1/4 inch works for palm sized items, but you can go up to an inch for very large works.

Once the silicone has cured, you can make a support shell out of plaster bandages applied a couple of layers thick. This "mother" mold will hold the flexible silicone in place during casting. Again, the bigger the mold, the thicker the mother.

Though it is probably not necessary, I like to give my newly made molds overnight to finish curing before I start casting in them.

If you feel you need to thin silicone caulking, xylene is the solvent of choice, but work outdoors and protect yourself from the vapors, they can cause health problems, so read the label. Mineral spirits will also work, but weaken the material and leave it with a greasy feel. Mineral spirits will also slow the cure from hours to possibly days.
Also, volatile solvents will result in shrinkage of the finished mold in proportion to the amount of solvent added to the silicone.

I personally like dry mold releases rather than greasy ones. For porous materials like plaster, I like to buff in several coats of Johnson's paste wax, leaving only a micro thin layer on the surface and then allowing it to completely haze over before making the mold.
For non-pourous surfaces, like glass, I take Ivory dish soap and mix it 1:3 with distilled water. Use a soft artist's brush and swish the sudsy mix over the surface of the model while drying with a blow drier in the other hand. The result should be a thin soap film that is relatively dry to the touch.
Petroleum jelly (Vasoline) will also work, but is messy to clean up. I don't like PAM or other cooking sprays.
Silicone sprays make great mold releases for everything EXCEPT silicone. They tend to become part of the mold and may actually increase adhesion.
Also avoid petroleum based lubricating sprays. They are messy and make it difficult to brush the silicone over the surface evenly.
For really difficult releases, there is a material called PVA (polyvinyl alcohol) sold in craft stores that sell acrylic casting kits used for making clear paperweights and such. Brush it on, let it dry, and then apply a thin film of paste wax. If that won't release it, nothing will.
PVA is soluable in water, so don't try to use the soap film for the secondary release.

When making a cast in a silicone mold, a release often isn't necessary as not much sticks to silicone, except more silicone. If you feel you must use a release anyway, silicone spray lube, soap, paste wax, or PVA will all work.

*** UPDATE *** - Monday 9 April 2007 I received an email from a guy in Vietnam that had some information to share on the subject of using POP (plaster of paris) to aid in the curing of silicone caulk. I thought I would pass it along.

While going through patentsite to look for a cure that works as well when the silicone is applied in thick layers, I came across the notion that Calcium Carbonate releases water when in contact with acetic acid.
Plaster is some sort of calcium carbonate.

I wanted a silicon object with a diameter of about 60mm using a plaster mould. (it is not what you think) I used a carnauba furniture wax as release agent.

I have mixed quickly the plaster and silicone (1:6 by weight) and spooned it in the plaster mold with a spatula taking care of air pockets. After about 5 minutes the whole mixture starts to thicken and after 4 hours I took it out of the mould without any problem. It was still smelling acetic but solid through and through. Now after 2 days the smell is gone.

Hope some can use this trick.


That out of the way, let's see what can be done with it. Click on the links below to go to that project.

A Silicone Caulk Glove Mold by Joe Hildreth