Explain it to me: Casting a Glass Hawk Head

Early this summer I had a chance to do something out of the norm. As part of René Lalique: Enchanted by Glass, our show about René Lalique here at the Museum, I had the chance (and challenge) to make samples for the hands-on activities located at Explainer carts in the galleries.

Museum Explainers, the Museum’s high school and college student program, work throughout the galleries during the summer providing hands-on experiences and answering questions about glass and glassmaking at carts stationed throughout the glass galleries. They are extensively trained in the spring to learn all about glass. The purpose of Explainer carts is to help visitors to the Museum better understand glass history and glass processes. For the cart in the Lalique exhibition, we needed an example of glass casting created through a process called “lost wax” or “cire perdue.”

Here is a bare bones description about how I got from this . . .

Clay Positive of Hawk Head

Clay Positive of Hawk Head

To this . . .

Glass Hawk Head

Glass Hawk Head

I started with an oil based clay and created a sculpture of a bird. I based it off of this piece in our collection which is currently on display in the Lalique exhibition here at the Museum (Lalique’s is much nicer).

Alginate mold from clay positive

Alginate mold from clay positive

From that clay bird, I created an alginate mold. Many of you have experienced alginate before – it’s what the dentist uses to take a mold off your teeth! Alginate is a dried and ground seaweed that when rehydrated becomes a liquid that quickly turns into a flexible solid.

I then poured wax into the alginate mold. The resulting wax was a close copy of the clay and I could now make multiple waxes from the same alginate mold.

I then cleaned up the waxes.

Wax positives

Wax positives

I wanted the positives to look as good as possible before casting them – changing the material into glass won’t hide any of the imperfections in the mold, so it is best to get them out at this step while still using a malleable material.

Once I had the wax positive up to snuff, they are reinvested in a mixture of plaster and silica flour. This is another material that goes from being a liquid to a solid, however, plaster and silica are ridged and capable of withstanding the high temperatures required to melt glass whereas the alginate material is not.

Plaster mold with the wax removed

Plaster and silica mold with the wax removed

With the plaster hardened, the molds are placed on a steam table. Heat from the steam melts the wax and it flows out of the mold. With the wax steamed out, I am left with a bird-shaped hole in my plaster mold. This bird-shaped hole can be filled in a variety of ways. Our sample is solid cast glass; the entire cavity was filled with glass and the object is solid. With this type of mold you could also create a finished glass object using the pâte de verre technique, where you would fill the cavity with a glass paste to add color in specific places (watch a video). There is also a third option—you could blow a glass bubble directly into the cavity and end up with the same bird shape, but hollow—much like the cire perdue vessels in our Lalique exhibition.

The casting process includes many steps, and, as you can see, it involves switching from positive to negative and back to positive and then back to negative. This complexity made it a good process for the Explainers to demonstrate because it can be so challenging to conceptualize.

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Hopefully this post helps to reveal a little of the mystery of the glass casting process. Multiple pieces in the Lalique show were created with a variation of this technique, see them for yourself before January 4th!

4 comments » Write a comment

  1. Hi towdesign,
    From Jessi:
    I believe that you are speaking
    to the section of the text where I state that “Lalique’s is much nicer” in
    reference to the Lalique hood ornament I based my hawk head on. In that comment
    I was considering the aesthetic qualities of Lalique’s hawks head in comparison
    to mine – I think that the reference piece from our collection has a much more
    streamlined feel and the proportions seem more pleasing to me.
    As far as the learning curve in
    cast glass it takes time to learn all of the skill necessary for a successful
    casting, even once you have learned the basics you will still be constantly
    refining your technique. For me the hardest part is waiting for the glass to
    cool before breaking it out of the mold.

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