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Saturday, October 3, 2009

Mold Making 101

I cast a lot of my glass works in plaster/silica molds. I make a model in clay first and this model must have flat bottom. I take a sheet or two of very heavy duty plastic and place it on a flat board and then set the clay model firmly on top of it. I then make a circle using bendable aluminum flashing (taped or clipped into a circle shape) and pull the plastic up though the top edge of the aluminum. I make certain the plastic is flat and even at the bottom of the circle. The aluminum circle must be a good inch or inch and a half larger than the clay mold. These steps should prevent leaks of the plaster/silica mix. I know some artists simply build clay dams around their models vs. using flashing and plastic but it can be time consuming and it is prone to leaking.

I then fill up a container with warm water and mix an equal volume of a 50/50 pottery plaster and silica mix to the water. It is best to keep a separate bin where you have already premixed the dry plaster/silica mix. To do so, I take a large bin and mix in a cup of each powder at a time. So I always have premixed dry mix on hand to make molds.

The plaster/silica powder is then dumped a cup at a time into the water until it begins to not sink into the water. The volume is typically 50% water and 50% dry mix and when in doubt, add more powder, not less. I next mix the lumps with my hands until the mixture is very creamy. I then begin to pour the mixture over the clay model until is about an inch higher than the model. It is good to sometimes agitate the mixture for a minute or two by tapping the sides gently to get rid of any air bubbles. Depending on the size of the mold, the plaster/silica should be set up in an hour to so.

I then remove the aluminum edging and flip the mold over. The clay must then be carefully removed from the mold. The clay can be saved for reuse later. After the clay is removed, I take a damp towel and further eliminate all clay residues. While you can now place crushed glass into the mold right away, I prefer to let my molds set for several days or more to eliminate additional water content.

When you decide to place your crushed glass into the mold, you have to stack it about a third or more beyond the height of the mold. The molds can only be used once in kiln-casting and after they cool, they easily crumble away from the cast glass.

If anyone has any questions, please feel free to post questions.

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