Making Your Own Molds Part 1

Open Face Molds

Being able to make your own molds opens a lot of doors for kiln artists. Although there are a lot of well designed commercial slumping and casting molds available, there are times when you just need to make your own special shape. In this series of articles I am going to address some of the key elements and basic concepts that relate to making your own molds.

Mold making is an art into itself and there have been many books written about the different processes. As kiln workers, we have some very specific mold requirements, primarily we need refractory mold materials that are able to withstand 1500 degrees F which other industries, such as injection plastics, don't have to deal with yet; we all have some things in common. I find it interesting to research different applications to see how they problem solved many of the same hurdles that glass artists deal with.

Which process and type of mold you use will depend upon your desired final outcome. Each mold material has areas where it shines and areas where it is not the best choice. A mold for slumping generally does not to have a lot of detail and there are a lot of options such as wet felt, stainless steel, ceramics, commercial kiln mold materials and cast insulating cement that can be used. Most of these materials can be used in the kiln many times.

If you plan on casting glass or doing pate-de-verre you may want to use a 50 / 50 mix of casting plaster or hyrdacal and silica flour. This receipt produces a mold that doesn't need to be coated with kiln wash so all of your detail will show. The 50 / 50 mix is basically a one time mold so plan on making a new mold for each piece of glass.

Cast Molds

Cast molds are just one of the many types of forms you can use in a kiln but they are popular because they can be used in multiple applications. The cast process is used primarily for making:

- hollow forms that glass can be slumped into,

- hollow forms that can be filled with frit that will be fired into a solid dimensional glass form,

- solid forms that sheet glass can slumped over.

Taking the time to learning how to make cast molds will open a lot of creative doors and it is an easy and fun way to get started as a mold maker.

Cast molds are made by pouring a liquid mold material into a container. The material sets up or hardens to produce the finished mold. Cast molds can be open face, which means that there is just one final mold needed to make the image or they can be multi-part molds that have two or more sections that come together to make the final mold. The multi-part molds are mainly used for vessels or ornate glass sculptures and they are a bit more involved, so lets start simple and focus on the open face molds for the time being. Open face molds are what you would use most of time for slumping but they are also good for casting.

No Undercuts

The one qualifier to keep in mind is that open face molds are "straight pull" molds and the image can't have any undercuts or areas where it might get pinched when trying to separate the glass from the mold. Sometimes the concept of the undercuts can be hard to grasp, I know it took me a while to totally get the hang of it. A poor choice would be to try and cast an object shaped like the number 8 because where the image gets narrow in the middle is an undercut. Let's place the 8 down into a container, fill the container with plaster and let the plaster harden. l8l

Can we pull the 8 straight up out of the plaster? No, because the center is narrower than the bottom so the 8 is trapped in the mold. Now if we did the same thing with an object shaped like the letter V, we wouldn't have any problem lifting it straight up out of the mold because it has tapered sides without any undercuts. That is what we need for open face molds - shapes that will pull straight out of the mold.

Negative and Positive

Part of the molds making vernacular are the terms negative (-) molds and positive (+) molds. If I want to make a casting of a small pyramid, the pyramid is considered the positive form (+). The mold that is made by casting plaster around the pyramid is a negative (-) mold. Negative (-) molds are the hollow molds. When working a sequence of different mold materials, the molds will alternate back and forth. A decision has to be made as to which type of mold we want to end up with. We slump into and cast frit into a negative (-) mold. We slump over a positive (+) mold.

How many steps?

There are two primary approaches to open face casting, a single step process and a multi-step process. An example of a single step process would be when we repair a broken panel from a bent panel lamp shade. We can take out one of the good panels and use that as our starting point or our "master model." We can make a negative (-) casting of the outside of the panel using our refractory (hi-temperature) mold material and slump a piece of glass into that mold. Because there is only one mold making step, we consider this a single step process. (In some situations it is best to make a positive (+) casting from the inside of the panel and bend the glass over the mold.)

Picture Caption - Each of these three face molds is made with a different mold material. This project stared by sculpting a Polyform Clay positive (+) from which a negative (-) plaster was cast. The negative (-) plaster was filled with a liquid RTV silicone to produce a flexible rubber positive (+) mold which can be cast over and over again.

You might have guessed that the multi-step process is a little more complicated. Because many of the refractory mold materials are only good for one or two firings, it is common to make a flexible rubber mold that can be recast over and over. Without a form to recast, you would have to sculpt the original master model again and again and that's no fun. Taking the time to make a rubber model allows you to make hundreds of copies of the image.

Let's work backwards. If we want to end up with a cast glass positive (+), we'll need a refectory negative (-) mold in the kiln. That negative (-) mold will be made by casting refractory over a rubber positive (+). The rubber positive (+) is made by pouring Room Temperature Vulcanizing (RTV) rubber into a plaster negative (-) mold. We use plaster because it is inexpensive and it not going to be in the kiln, so it doesn't need to be refectory. The plaster mold is a casting of the clay positive (+) master model. Now is that a multi-step process or what? To recap in the order in which they were made - Clay (+) / plaster (-) / rubber (+) / refractory (-) / glass (+).

I'm afraid that I might be creating more questions than I am answering but, unfortunately we are out of room for this issue, so be sure you pick up the Stained Glass News on a regular basis so you don't miss the next installment of my series of mold making articles.

Until Next Time,

Keep a warm Kiln,

Gil Reynolds