MOLDS FOR SLUMPING
Molds, sometimes spelled "moulds," are essential if you want your glass to take on a third dimension. They are most frequently used for glass slumping and shaping in the kiln. This chapter will focus on molds for slumping; information about molds for kiln casting is in the kiln casting section of the website.
Click on the particular topic or type of mold that interests you.
- Slumping versus draping
- Clay molds
- Stainless steel molds
- Cement molds
- Ceramic fiber molds, including "wet felt"
- Plaster/silica molds for slumping
- Found molds
- Drop rings
- Applying kiln wash to molds
- Applying kiln wash to stainless steel molds
Molds for slumping can be used in several different ways, but the most common are slumping INTO and slumping OVER the mold.
- Slumping INTO the mold. Probably the most widely used way to slump, this approach allows the glass to sag into the mold. Shapes frequently formed in this manner include bowls and platters. The molds used in this way need to have small holes in the bottom in order to allow the air to escape when the glass slumps.
- Slumping OVER the mold. This approach, in which the glass is allowed to fall over the outside of the mold, generally uses molds made of stainless steel. Because bowls and vases made using this technique tend to be characterized by their wavy sides (like folds of cloth), slumping over the mold is sometimes called "draping."
One very important thing to keep in mind is that molds that are used for slumping INTO (rather than OVER) need to have holes drilled in the bottom. This is to allow air to escape from the mold as the glass slumps into place. Without the holes, air will be trapped underneath the glass and it will not be able to slump properly.
Commercial molds for commonly slumped forms such as bowls and plates are widely available, as are the materials to make your own molds should you desire.
Molds can be made of materials as diverse as stainless steel, pottery clay, or plaster/silica mixtures. You can even use found objects, like auto hubcaps or terra cotta pots, as molds. Some molds need to be covered with kiln wash before they can be used; others are ready "as is."
Also, note that some materials do NOT make good molds. Copper and aluminum won't work; they will either melt or deform badly in the heat of the kiln. Steel sometimes works, but it is likely to oxidize or even warp. Cast iron may work, but it is very heavy and prone to rust. Stainless steel is by far the easiest and most available material to use for metal molds.
Most commercially available molds are made of either vitreous clay or stainless steel. Other kinds of molds generally need to be mixed and shaped by the artist.
Pottery clay can make excellent, long lasting molds. Clay is also inexpensive and widely available. If you buy a commercially manufactured mold, it should be good for dozens of firings. You'll just need to make certain you cover it well with kiln wash before using it the first time. Although some people advocate applying up to ten coats, just four coats with a haike or similar brush should suffice. Just brush on one coat in each compass direction. Reapply kiln wash when the previous application starts wearing thin.
Also, be aware that slumping inside the clay mold works best. Since clay shrinks less than glass (has a lower COE), slumping over the outside of a clay mold can sometimes cause the glass to crack as the mold cools.
Another thing to keep in mind is that you may need to slow down the rate of temperature change at around 1050 degrees Fahrenheit. This point, know as the "quartz inversion" temperature, is where clays tend to change the most in volume. Avoiding extreme temperature changes at this point helps reduce the likelihood of cracking the mold.
In many ways, stainless steel is the perfect mold material. It is lightweight, difficult to deform, and able to last for a thousand firings or more. Simple bowl forms are relatively inexpensive to buy -- you can even use cheap stainless steel bowls. (All you need to do is drill three or four small - 1/8" or less - holes in the bottom for air to escape.)
Stainless steel molds do need to be covered with kiln wash. This is difficult to do when the mold is at room temperature, but it can easily be accomplished by heating the mold to around 400 to 500 degrees Fahrenheit, then brushing or spraying on the kiln wash while the mold is hot. The water in the wash will evaporate, leaving the protective elements behind.
Also, it's important to realize that steel contracts more than does glass. (That's the opposite of clay, which contracts less than does glass.) As a result, slumping on the outside of a stainless steel form generally works better than slumping on the inside.
Still, you can get away with slumping inside gentle bowl forms; just make certain it's well covered in kiln wash (you might even sprinkle a little kiln wash powder inside) and be aware that slumping inside deeper forms (such as inside a stainless steel cup) may not work.
There are three major types of cement molds: castable cements, insulating cements, and gypsum cements. All are dry mixtures to which you add water.
They are usually used to make a form for slumping over or for pate de verre (filling a mold with glass frit and then firing to fusing temperatures). They tend to stick to the model being used, so the model usually needs to be coated with a parting agent such as Pam cooking spray or liquid soap. Insulating cements tend to be less dense than the other types of cements.
Both castable cements and insulating cements require that kiln wash be applied to the finished mold to make sure that the glass doesn't stick. By contrast, gypsum cements do not need kiln washing. They are also able to pick up more detail than the other kinds of cements.
There are two major types of gypsum cements: hydrocal and hydroperm. They both work very well for casting into as well as for slumping over or for pate de verre. Hydrocal tends not to be as strong as hydroperm.
Ceramic fiber materials come in many varieties. The best known of these is probably fiber paper, commonly used to line the kiln shelf to help prevent glass from sticking to the shelf. Other types of ceramic fiber materials include fiber blanket, fiberboard, Fiberfrax, and the Luminar line of products. All have in common that they are composed of ceramic fibers containing alumina, silica, or similar components.
Not only do these products resist the heat of the kiln, they also can be treated with rigidizing solutions to give them strength and permit them to function as molds.
"Wet felt," for example, (also called "moldable fiber blanket") is a fiber blanket which has been treated with a rigidizer and can be used as a mold by simply molding it to the model or shape to be duplicated. When it air dries, it can be removed from the model and used as a slumping or other warm glass mold.
People who work with clay and similar materials often use plaster to make their molds. Pottery plaster, which is made of gypsum, has the advantages of being fairly easy to find, easy to work with, and relatively inexpensive. By itself, however, it won't work for making molds for warm glass work.
That's because plaster alone can't withstand the heat of the kiln. In order to keep it from deteriorating when it's fired, you need to add one or more refractory materials to the mixture. (A refractory material is one that doesn't burn or disintegrate when exposed to heat.)
The most common refractory material to use with plaster is silica. Silica, available in both sand and flour (very small particles) formulations, does not deteriorate at fusing and slumping temperatures. That makes it an ideal companion for plaster. Plaster/silica mixtures are also safe to use without kiln wash. It's no wonder they're frequently used for pate de verre and casting molds as well as custom slumping forms.
Despite their usefulness, making plaster/silica molds is not as simple as making many other types of molds. Proper mixing requires practice and specific safety procedures must be followed.
Unlike most molds, which are either slumped into or over, drop rings involve slumping THROUGH the mold. This is possible because the drop ring, which is most often made of a ceramic material or cut from fiberboard, has a large hole in the center.
To use the ring, it is supported by kiln posts so that it's above the kiln shelf. The glass, which is placed on top of the ring, slumps through the hole when it is heated. With careful firing, it will stretch until it reaches the kiln shelf that is usually placed about six inches below. Doing this right requires patience and close examination of the glass as it slumps.
Smaller drop rings made of clay are commercially available, but larger ones must be custom made. This is generally done by cutting them from fiber board.
You'd be surprised how many found items can serve as molds. We've already mentioned inexpensive stainless steel bowls and old pottery, but molds can be made from rocks, from steel hubcaps, from old tuna cans, or from just about anything that can withstand the heat of the kiln.
Just be sure that you use kiln wash on the found item and that you heat it slowly in an initial firing until you're certain how it will behave in the kiln. Some items, such as rocks, can have water trapped inside and could explode or fracture if heated too quickly.
But just because a found item can't stand the heat, it might still be worth using as a form around which to cast a more usable mold using clay or another refractory material. So keep your eyes peeled for shapes to use as molds -- you never know what you might find.