Glass casting is the process of melting glass into a mold where it solidifies to create a sculpture. The technique dates back to the Egyptian period. In modern glass art, the most common glass casting techniques are kiln casting and casting into sand.
The glass casting technique I use to create my glass sculptures is kiln casting. These are the steps I follow:
How to cast glass: 7 steps
1. Design your sculpture
The first step in the glass casting process involves drawing your idea — you can skip this step if you are a fan of free flow — and then shaping it using a material like clay or wax. I use a specific modeling clay that doesn’t dry out; this way I can take my time and don’t have to worry about keeping it moist.
If you shape your model in wax, you will need to melt out the wax after you make the mold (this technique is known as lost wax casting). I have used this technique too but my favorite material is modeling clay.
You can also use other materials such as styrofoam or, depending on the shape you want to achieve, you can also combine modeling clay with styrofoam.
Tip: Any material you use must be compatible with plaster. You’ll need to remove the prototype from the investment mold (see step 3) and the last thing you want is a material that glues to the investment!
2. Make the plaster mold
This is when things get messy. Once you have your model ready, you need to build a wall around it. As you can see in the picture, I use non-porous melamine panels and attach the walls with clamps and screws on the sides to make sure no investment will escape when I pour the mix inside the cavity.
Once you have assembled the melamine panels, use a brush to coat the inside of the walls with a release agent to prevent the plaster from sticking to the mold’s surface.
When the walls around your prototype are ready, the next step is to make your own investment mix (or buy the mix already done). To make your own mix, I use a mixture of hard plaster (50%) and silica powder – quartz (50%). This is the mathematical formula I use to calculate how much water I’ll need for the mix based on the size of the prototype (you can find a more detailed explanation for your investment calculations in this other post):
(Height x width x depth / 2) x 0.64
After mixing the plaster and silica powder, you’ll need a big bucket or plastic container where you’ll pour the water and start adding the plaster mix very slowly. You can stir the mix with your hands (use long gloves!) or use a drill with a stirring attachment.
The next step is to start pouring the mix inside the cavity and slowly cover the prototype. You’ll need enough mix to cover the full prototype and add a couple of extra inches of liquid once it’s covered.
Warning: If you haven’t assembled the walls properly the plaster liquid could escape and make a big mess. The best solution to avoid this, besides proper clamping, is to seal all the outside and inside panel junctures with clay.
To avoid trapped air in the mix, once you’ve poured all the investment inside the cavity you can use an air compressor to get rid of any bubbles you may see on the surface.
Tip: Always remember to use an FFP3 mask during this part of the process – over time, silica dust particles can cause serious lung issues.
3. Remove the model from the mold
After a few days, the investment mix should be dry enough and ready for the next step of the process. You can then remove the clamps and the 4 melamine walls and carefully flip the investment mold. If you have used clay for your prototype, remove it slowly to avoid breaking any of the plaster walls (you can use clay tools like the one in the picture).
If you have used wax to make your model, in this step you would put the mold facing down and melt out the wax with a heating tool.
Once the prototype is removed, clean the mold thoroughly until it’s impeccable and there are no unwelcome particles. Let the mold dry completely (this may take a few days depending on the size of your mold and humidity).
Tip: As this part of the process involves destroying your model, you may want to consider making a silicon mold after step 1 to have a backup of your prototype.
4. Calculate the glass you’ll need
An easy way to calculate how much glass you will need for your sculpture is to pour water into the mold until it’s full and then measure the total volume of water you have used. You can do this by pouring the water back into a container and weighing it or just weighing the water before you pour it inside the mold. Just make sure you only spend a few minutes doing this so that the mold doesn’t get wet for too long.
Once you have the total volume of water (in grams), you need to multiply this weight by 2.5 (the density of glass) to calculate the total volume of glass you’ll need for your sculpture.
You can use glass billets or alternative glass shapes to obtain specific patterns inside the glass sculpture. The type of glass you use will determine your results. I use optical glass from Ohara to achieve crystal clear transparency.
Tip: Precision and experimentation in this step can make a big difference if your main focus is on what happens inside the sculpture instead of external shapes.
5. Place the glass inside the mold
Place the empty mold inside the kiln. Make sure it’s impeccably clean before you put it inside the kiln as any tiny particles could be an issue.
In a separate area of the studio, clean all the surfaces of the glass with diluted white vinegar or distilled water. Use gloves to avoid fingerprints in the glass. Once all the glass is clean, place it inside the mold in the kiln and get ready for firing.
Tip: Never use soap, alcohol, or other agents that contain ammonia to clean the glass as these can promote devitrification.
6. Fire the glass in the kiln
The firing temperatures will vary based on the glass you use and the size of your sculpture (the thicker the piece, the longer it takes). You will need to adapt your firing schedule to the specific type of glass and its technical particularities (softening point, annealing point, and strain point).
For my optical glass sculptures, my firing schedule may take up to 2 weeks: a couple of days to melt the piece up to 850ºC, and then several days of annealing and cooling down.
Once the firing schedule is completed and the kiln reaches room temperature you can break the mold with a rubber hammer (my favorite part), remove the sculpture from the kiln and start the polishing process.
Tip: The most critical part of the firing schedule is the annealing stage. You should bring the temperature down as slowly as possible during this stage. If you don’t do this properly, the glass can crack or shatter.
7. Polishing your sculpture
When a glass sculpture is fired in the kiln, the result is an opaque surface. The piece needs to undergo several stages of grinding and polishing to achieve optical transparency.
To learn about all the polishing steps, you can read my other post on how to polish glass to achieve optical transparency.