Friday, August 17, 2012

how we do the thing we do

One of my readers commented that he didn't understand the process of my work and although I have a very detailed explanation on my website, I thought I would describe it here.

Pate de verre, literally translated means 'paste of glass', is just one of several kiln casting techniques. I say kiln casting because the work we do is formed in a kiln and not with molten glass out of a furnace or formed on the end of a blow pipe.

The process as we developed it is a lost wax process similar to bronze and precious metal casting, like jewelry. Other artists use clay for their models and some cast from life, burning out the organic material in the kiln before adding the glass to the mold. Our models are sculpted in wax. A model is an exact replica of the thing you wish to cast in glass.

Pate de verre is one of the oldest known forms of glass forming, going back 3,500 years or so. The name for this technique comes from a revival by french artists in the late 19th century. If you are interested in a more detailed history of glass forming in general and pate de verre in particular, you can read about it here.

The first step to any piece is to make the model. Sometimes I carve the item out of a block of wax (like the peach inlay piece), sometimes I build it up with shapes cut out of wax made into 1/8” thick 'sheets' and fill in the hollow areas with small bits of wax (like the flower sculpture), and sometimes I use reproduction molds that I have made from an object (like the latex mold of the peach pit) that I pour melted wax into. For the box itself, we made thick slabs of wax to specific dimensions and I joined them together.

wax carving in progress

Once the model is finished, it is glued down to a surface to keep it from floating up when the investment, or mold material, is poured over it. When the glue has dried, we make a dam, or coddle, around the model leaving an inch or two of space between the model and the sides of the coddle.

set up for investment

Our investment, or mold material, is composed of plaster powder and silica flour and water. When mixed, it is poured into the space between the model and the coddle until it has covered the model completely to a thickness equal to that of the space around the model. When the investment has hardened, then the wax is melted out, hence the term 'lost wax', leaving a negative space in the now hard investment material. This is the casting mold.

pouring investment around the model

finished mold

The next step is to add the glass into the mold. Our technique, pate de verre, uses glass that is crushed to the consistency of sand, called 'frit' or powder. I mix the frit and powder with a binding agent or glue to make a 'paste' of the glass and pack the different colored glass pastes into the negative spaces of the mold where I want the colors to be in the fired piece. This process can be very tedious and take many hours.

placing the glass 'paste' in the mold

Once all the necessary glass is in the mold, it goes in the kiln for a 3 – 5 day firing. It must be heated up slowly, held at casting temperature for a specific amount of time, and then cooled down very slowly. Going too fast up or down results in either a cracked mold or a cracked piece. We have no control, really, once it is in the kiln, beyond programming the firing schedule.

After the firing schedule is completed and the inside of the kiln has returned to the ambient temperature, the mold is removed from the kiln. After firing the previously strong plaster/silica mold is now very soft and easily breaks away from the cast glass inside.

removing the investment material

After the plaster/silica residue has been removed from the surface of the piece, the final finish work is done with small diamond bits and polishing compounds. Some pieces need a lot of finish work and some hardly need any at all.

doing the finish work

We never know until it comes out of the kiln if everything went as planned. When we first started out we had a failure rate that exceeded 50%. Now, it's about 10%.

This process is probably the most tedious, time consuming, detail oriented, and frustrating thing I have ever done but it is also one of the most rewarding when it all comes together and comes out right. It's also the only process that allows for specific placement of color in the mold and the finished piece and the only process that gives the finished work it's distinctive luster when using transparent glasses.

a finished piece


  1. thanks for walking us through this! it is fascinating for those of us not familiar with any of it.

    and WOW! talk about a TIME investment in your work!

  2. I sort of knew how the process went, but this was really fascinating. I can't even imagine having the patience for that type of work.

  3. Ellen, this is spectacular. I never knew of such a glass process. I love glass works. They always capture me. I have a couple of Satavas. Brightened my day. Thanks.

  4. What a process. You do such skilled, beautiful work. I'm always so impressed.

  5. Interesting! I'm glad to see how all this works. I guess I had some vague idea what you were doing, but definitely not the step-by-step process.

    Your comments about never knowing how something will turn out remind me of my days doing pottery. Same thing -- you put it in the kiln, and it's out of your hands!

  6. I think glass is one of the most beautiful materials
    the look, feel of it is wonderful
    I love your work

  7. Thanks for your step bye step! Your work is so beautiful & the finished pieces are amazing. I'm working on a series of eyes but I'm at about the 50% success/failure rate. My molds almost always crack. Thank you for sharing!

  8. Thank you very much Ms.Ellen.
    Such an intricate process. I understand a bit better. We use a "molding" process with our caststone,,,but,,,the beauty in your finished products is exceptional...thanks, glenn

  9. Glass is so fascinating. A beautiful medium to work with, and it must be like magic when the piece finally emerges. Magic at least 90% of the time.

  10. Elaborate and enthralling... what do you do with the wax after you are done?


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