What does a casting look like after firing? Removing the plaster and excess glass.

As I mentioned earlier, the recycled glass is kiln-cast in plaster and silica molds. After firing and cooling, the plaster will easily crumble away from the casting. Sharp glass edges may remain on some of the casting so wear gloves.


Most castings do not come out perfect and a lot of cold working must be done. I first use use my glass grozier pliers which is a hand tool for removing away small excess bits of glass. Since you have to estimate how much glass to stack in your mold, excess glass can flow around the edges of the mold and this glass can be nipped away. If the glass is too thick, it will have to be cut off with a wet tile saw or glass saw (electric).

The casting has to be wet scrubbed with a variety of brushes to remove the plaster bits embedded in the glass. Any pointed tool will also work in picking out bits of plaster. There will still be some pinpoint plaster bits remaining so the casting can be soaked in a variety of substances. I have had some luck with Coca-cola which acts to fizzle the plaster out of crevices. Lime-Away can be used as well but since this is a toxic chemical, wear gloves and do not leave it unattended where children or pets may have access to it. Some chemicals can create an etched effect so be cautious in testing different chemicals if you do not want to achieve that effect.

Freecycle: A large donation of four bags of bottles!

One source of recycled glass and other materials is Freecycle.org. You can go to the website at www. freecycle.org and sign up for the freecycle in your area. I think Freecycle limits you to two cities/areas so choose wisely. People post unwanted items that include everything from sofas to toys to even glass bottles or a broken glass table top on occasion. The only requirement is that you have to pick it up (assuming you email back your interest and you are selected by the poster). You can also post requests for items you are seeking. While large quantities of glass do not pop up every day, I usually come across something I need monthly.

Someone recently posted that they were giving away multiple bags of wine bottles. I asked if they were from varied products or one to two identical products. The bottles were huge and came from 2 identical sources. While this is no guarantee of compatibility kiln-wise, it does increase the likelihood a bit. I did some pretesting and the large set of 16 plus identical green bottles seem very compatible. The smaller set of about 8 smaller brighter green smaller bottles did not seem as compatible between bottles so they will not be mixed together in kiln-castings.

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.