Casting Glass in an Electric Kiln Using recycled glass to cast in plaster molds

Developing a Clay Positive When developing a clay form for the mold there are items that must be followed. –Keep the form small, remember that the form must be invested in plaster and weight and size is a concern. –No sharp edges or under cuts. Remember that the Glass will flow better around smooth curved edges that share edges. Any sharp under cuts will cause problems when trying to clean out the clay from the plaster mold and these areas will create air pockets that stops the glass from filling those sports. –Sharp under cuts are where the object turns up quickly when the form is held upside down, keep in mind that the object will be cast upside down.

Sharp Undercuts The glass can not follow up into these areas.

Cast Glass »Kim Perrier

Casting Glass in an Electric Kiln All clay positive must be checked by the instructor before any one starts setting up for their plaster casting. Once the teacher has signed off on the clay sculpture, students can mount their sculpture on the reservoir cone.

Reservoir The reservoir is a large clay cone which is added to the bottom of the clay positive. It will help during the mold making process and hold the glass when in kiln. This reservoir should be considered when designing the clay positive. Where are you going to mount it, side, top, bottom?

Cast Glass Brian Russell

Constructing Tar Paper Mold The clay positive should be firm but still moist when pouring the mold. Find a wood panel that leaves at least six inches of space around the clay positive. Place the reservoir in the center of the panel and make sure it sticks firmly to the panel. Tar paper will be used to help create the walls of the mold. In a circular shape, place two or three layers of tar paper at least four inches out from the clay positive and higher then the clay positive. Staple the tar paper together to hold the shape. In order to reinforce and hold the tar paper in place around the positive, small clay coils must be placed on the inside where the tar paper meets the wood panel. Push the coils down to create a seal inside. Brush on a thin layer of Vaseline on the inside where any wood is showing. This will prevent any plaster from sticking to the wood.

Pouring Tar Paper Mold Use a plastic bucket to mix the plaster, silica, and fiberglass paste. (A mask that covers your mouth and nose should be used when mixing.) Fill the bucket with water. The amount of water depends on the size of your mold and the amount should be about half the size of the predicted mold. Then cup by cup gently sift the plaster then silica into the water. Do not mix until the plaster/silica powder is settling on the top. Add a couple of handfuls of fiberglass into the mixture. Mix the ingredients together until it is smooth and consistent. Now the mix is ready to be poured into the tar paper. Do not pour onto the clay positive but on the side. The mixture should go at least two inches above the positive. Shake the mold or the table gently in order to get all the air bubbles out of the mixture. You must let the mold dry for twenty-four hours before taking it apart.

Taking Apart Mold and Digging Out Clay Gently break through the outside layer of plaster and then remove the tar paper. Before turning the mold over, shave the edges down till they are smooth. Turn the mold over and remove the clay from the outer edge and shave the edges smooth. Smoothing the edges prevents the mold from cracking when being fired. Now the clay positive must be removed. Be careful not to scrape the mold with the tools when doing this. Start by taking as much clay out of the center as possible. Then slowly start peeling it away from the walls until all the clay is gone. Use a wet sponge to remove the excess clay residue from the mold. Do not scrub to hard or use too much water because details will be lost.

Mold This is a mold ready to be cast

Displacement Test Step one – Find a good-sized clear container and fill it about three quarters full of water. Mark the water level on the outside of the container with a waterproof marker. Step two – Take the water and poor it into the mold until it is at the desired level you want the glass to be, remember any extra glass will have to be ground off. Then quickly dump out the water that is still in the mold. Step three – Take chunks of clean glass and start to fill the container until it reaches the marked line. Once you reach the line stop and dry off the glass chunks. Step four – After the glass chunks are dry, place them into your mold in an orderly fashion.

The Glass Casting Process When casting glass it is important that air flows inside the kiln to properly fire the molds. First place a ceramic shelf on kiln bricks. Allow there to be some space between the wall and the shelf on all sides of the kiln. This will allow the air to flow underneath and melt all of the glass at an even rate. When placing molds in the kiln do not set the molds within an inch of the heating elements or within an inch of each other. This allows airflow on all sides and increases success in the firing process. Setting them to close can cause the mold to break in the firing process. Do not set another kiln shelf above the first, it will restrict airflow.

Firing the Kiln When firing plaster silica molds bring the temperature up slowly. Every kiln is different. It is important to run a few experimental runs to find a correct program for the kiln. Hold the kiln at 200*F to dry out the molds. This helps prevent the molds from cracking as they reach higher temperatures. Slowly raise the kiln to 1475* and hold for about an hour. At this temperature the glass viscosity begins to decrease and begins to fill the mold. Continue to increase the temperature to 1500* and hold the kiln there until the melting process is completed, around three hours. Up until this point it is fine and recommended to check the kiln periodically to make sure everything is going smoothly.

Cast Glass Eurhythmia

Annealing Annealing is the process in which the glass cools and stabilizes in a solid state. It takes place in the upper temperature of the cooling process. It is important to lower the kiln carefully for many factors. If the glass is fired for too long it will show signs of diversification. Inadequate cooling can also produce internal instability and can cause the glass stress and ultimately to crack. Annealing also relies on the size and thickness of the casting. The thicker and larger the piece, the longer it takes to anneal. Included are a couple of annealing charts to give a better idea of how long to fire and anneal the glass at different thicknesses. Another place to get annealing information on glass is the website where the glass was purchased. Both Bullseye and olympic Color Rods have this info on their website.

Removing Mold from Glass Gently break through the outside layer of plaster. Gently brush the surface with a stiff nylon brush. Wash with water to remove dust and then start the cleaning process with the Cold working tools.

Cold working Tools The ideal machine to use would be a stationary belt sander. Using different belts that go from coarser to smoother also help make the piece look finished and completed. It is possible to complete this task by two other hand-driven less expressive methods. 1. A special set of diamond pads can be used to grind down the unwanted flash. A set of four can be purchased for about $ Each pad has a different grits of diamonds. These can be purchased at glasscolor.com or bullseyeglass.com. 2. The other method that could be used is sand paper. I would recommend starting at an extremely coarse grit of paper first and work down finer and finer grits at a time. This process will require quite a bit of patience. The more put into it the better the results.

Links to more information on Glass Casting Hugh Mckay Glass casting with clay molds glass castig panels glass bowls