Introduction: Fusing and Slumping Glass

In one of my sculpture classes at Carthage College, my students and I experimented with fusing and slumping glass. Carthage had many glass supplies, a kiln that looked barely used but no one teaching it. None of us had ever done this but I felt with my experience enameling, metal working and casting, we could do some research and we could figure a few things out.

Step 1: Capturing the Image

For our assignment we started with 3D scans generated from the Sense 3D scanner. I used this scanner because it is inexpensive, very simple to use and is great for introducing beginners to digital fabrication. There are many tutorials out there on this and it is so easy to use that I do not feel it is important to go through this step here.

I generated G-code using Rhino and Rhino CAM and cut each student's face in the negative into a piece of 2" pink foam on the CNC router at Frankie Flood'sDCRL at UWM. These processes are complicated and not the scope of this Instructable so I will only be showing brief documentation on this.

Keep in mind, you do not have to use CNC processes for any of the glass slumping, simply sculpt something directly in clay with draft and skip the next step.

Step 2: Cutting a Pattern

The next step is to create a pattern to slump glass into or over.

We started with 2" pink foam for our 1st pattern and then used a plaster/silica mix to make a mold and generate our final pattern for slumping. I did a fast, rough cut on the CNC router with a 3/8" flat end, 2 flute end mill on the foam and then switched to a 1/4" ball end mill for a second pass to pull out some detail.

When the foam "pattern" was finished we need to decide how we were going to slump the glass before proceeding. For our experiment, we tried slumping the glass two different ways and I need to experiment more before suggesting the best way. First, we slumped the glass IN a mold with the face being a negative in the mold. The glass did not flow well into the nose and the surface that came into contact with the mold had a matte surface so we decided to re-slump it OVER a positive form. In both cases, we need to make a mold.

Step 3: Preparing the Mold for Plaster

If we were to pour our plaster/silica mix into our negative foam carving, we will have a positive pattern we can then slump glass OVER or on top of. However, if we want to slump our glass INTO a negative pattern, we need to start the mold making process with a positive form.

To begin, press a 1" slab of water-based clay into the details of the negative pink foam shown above. Fill the rest of the mold with clay and flip it over. Work out the clay face and touch up any details or changes with clay tools. The student featured here (Tori) did not want to capture her hair so it was cut off the clay version before further mold making. Set the solid positive clay face on a board (face up) that is several inches larger in diameter than the clay face. Make sure the board either already has a resist such as a laminate, or apply one such as clay water or Vaseline. Next, build a 3" tall clay wall approximately 1-2" away from the face. This will act as a retaining wall so we can pour our plaster/silica mixture directly over the face and contain the fluid mix around it. We do not need a release for the clay, but just make sure the board has one.

Step 4: Mixing and Pouring the Plaster/silica Mold

Below is information taken from Jennifer Halvorson: Assistant Professor of Glass at Ball State University on mixing the plaster/silica for slumping glass.:

Plaster/Silica Formula

There are many different investment formulas, but the simplest uses equal parts of silica flour (around 200 mesh), pottery plaster, and water. This produces a mold that is inexpensive and will work fairly well for small projects. Start with a clean container. Using a container that has not been thoroughly cleaned may contaminate the mixture or cause it to set up more quickly than desired.

Normally, the plaster used can be any of the commonly available pottery plasters. Even plasters such as Plaster of Paris will work well for most purposes. Silica, which is a hazardous material that can cause silicosis (a lung disease), is generally used in powder form. 200 mesh works well. In some places ground silica is also known as flint or silica flint. You should wear a respirator or approved mask when working with silica to keep from inhaling the potentially harmful silica dust.

Mixing the Plaster/Silica

Start the actual mixing process by adding the plaster/silica mixture it to the water. (Always add the dry ingredients to the water, not the water to the dry plaster and silica.) Add the dry mixture a little at a time. Some people use a sifter to evenly spread the dry mixture on the surface of the water, but simply sprinkling the particles across the surface will work also.

Continue adding the investment until it "peaks." Peaking occurs when the mixture sinks slowly and dry investment islands appear in the container. (If you measured out your ingredients properly, the mixture should "peak" just as you use the last of the dry ingredients.)

Once the investment peaks, mix in all the dry ingredients until the formula is a consistent thickness. Then stop mixing and allow the mixture to sit undisturbed for about five minutes. This process, called "slaking," helps ensure that the investment particles become saturated with water.

Pouring the Mold

As the mixture slakes, it should become thicker. Watch carefully until it begins to thicken, then slowly pour it into the mold box that surrounds your model. To minimize bubbles and distortion of the model, pour the investment along the edges of the box, rather than directly on the model. Do not pour leftover mixture down the sink, as it will harden and ruin your plumbing.

After the mold has air-dried, you should be able to remove the model and allow it to further dry for at least 24 hours.

Step 5: Curing the Mold

Below is information taken from Jennifer Halvorson: Assistant Professor of Glass at Ball State University on curing the plaster/silica for slumping glass.:

When a plaster/silica mold is heated, the mold goes through three phases. Regardless of the size or shape of the mold, it's important to recognize each of these phases and to soak the mold at each phase so that the mold will dry without causing cracking or other distortion.

The first phase is at around 225 degrees F (107 C). At this temperature physically absorbed water within the mold can be driven off, so that the process of drying the mold can take place. For small molds (under 8 inches/20 cm) it's sufficient to soak a wet mold for around two hours to drive off physically absorbed water.

The second phase, which involves the removal of chemically bound water in the plaster, takes place at around 350 F (177 C). A two hour soak will generally work well to remove chemically bound water from small molds.

The final critical phase occurs in the temperature range from 500 to 850 F (260 to 450 C). This is where the plaster starts to shrink as it completes the drying process. It's important to fire slowly (take about three hours for small molds) through this range to avoid cracking the plaster or fracturing the mold. For larger pieces the firing times and soak times will need to increase.

Step 6: Shelf Primer

In order for the glass to not stick to the mold, you will need to use a kiln wash or shelf primer. The primer we used was in a powder form and it is mixed with water; equal parts by volume. It will be applied to both the shelf if you are fusing glass and the mold if you are slumping glass. We applied it with a brush and cured it in the kiln at 550F for 20-30 min before allowing it to cool and begin the slumping/fusing process. This primer changes color from blue to white/grey indicating it has cured.

Step 7: Fusing Glass

If you just want to slump one color, or a single plane of glass, you can move onto the next step "Slumping Glass". Here, we will go through the process of fusing two or more pieces together. After this, we can slump our new piece of fused glass into our plaster/silica mold.

To start, we need to decide the dimensions of our finished piece. At school, we have several bins of various shapes, sizes and colors of glass. The pictures above show Tori working on a design consisting of many fragments and shattered pieces of glass on one larger sheet. Once the design is finalized we laid the piece on the primed shelf and set the kiln.

Here is the basic guidelines we followed:

1 - 400F(degrees per hour) - 1225F - then hold for 30min

2 - 600F(degrees per hour) - 1490F - then hold for 10min

3 - cool to 900F (at kiln's natural rate) -then hold for 1hr

6 - cool to 700F (100F per hour)

4 - cool to 70F (at 277F per hour)

The second picture above shows what happens when you do not cure the primer on the shelf before putting the glass down. Since we did not cure the primer first, it cured under glass, releasing gasses and "fuming" from the shelf. I peaked several times because it was the first time I had done this and I saw large bubbles the size of a golf ball creating the holes seen above. Although it messed up the piece she was going to use for her mold, it created some great results for a different application. If you do not want holes, follow the previous instructions on "Shelf Primer".

Step 8: Slumping Glass

After the primed mold has been cured and the glass is ready, we set the glass over the mold and set the kiln. For our pieces we tried slumping our glass into a mold that had the negative of the students face. The 1st picture shows these molds that basically look like they pushed their faces in a bowl full of frosting.

These are the basic guidelines we followed:

1 - 300F (per hour) - 1225F - then hold for 10min

Visually confirm slumping

2 - cool to 900F (at kiln's natural rate) -then hold for 1hr

6 - cool to 700F (100F per hour)

4 - cool to 70F (at 277F per hour)

The glass did slump but did not flow into the nose or other details very well. In addition, the surface that comes into contact with the surface of the mold is slightly matte. I had to bump the time and the temperature up a little more than 10 min at 1225F.

We decided to create a second set of molds that were in the positive. So instead of the mold looking like someone pushed their face into it, the molds looked like the students faces were pushing out of the kiln shelf. We hovered the newly slumped glass over the molds on fire primed fire bricks. This allowed the mold to slowly slump evenly over the face. In addition to bumping up the temperature and time, I had to reach in with a metal rod to push the firebricks out of the way so the glass could fully cover the mold. The glass is not extremely malleable but enough to slowly slump; it is similar to acrylic at 300-400F.

In both cases there were issues that an experienced glass worker would easily overcome but this was about trying something new with the resources we had on hand.