Excellent glass casting tutorial from http://www.system96.com/Pages/CastingTutorial/billets.html

I didn't create or have anything to do with this tutorial below but I have discovered that when I really like something on the internet, I have to make some sort of copy of the information just in case it disappears later on and I can't find the information anymore.  This is a System 96 casting tutorial.   It is full of detail and very valuable information.  Hopefully you find this tutorial as wonderful and helpful as I do!

KILN CASTING A GLASS TABLE WITH FUSIBLE BILLETS

The August 2001 issue of Glass Craftsman magazine, presented an overview of how to create kiln-cast architectural slabs and sculpture using casting billets. Here are more detailed steps and explanations about the open face mold process. These steps describe creating a thick, cast glass slab 15"x15"x .75". This technique can be scaled up to much larger sizes with only minor adjustments in the annealing schedule.

Overview of this project

A clay positive is formed. A mold box is built around it. Mold mix is poured onto the clay positive and allowed to set up. The mold box is removed and the clay is removed from the plaster. The new plaster mold is dried and then filled with glass and glass elements. The elements can be frit, stringer, noodles, cane, billets, billet chunks or sheets. The plaster mold, filled with glass, is kiln fired then annealed. The plaster cast is broken away to remove the cast glass object from the mold.

Step #1 Form a clay positive

slab is size and shape of finished piece

A 3/4" thick slab of clay is rolled out for the positive model.

Place clay on a sheet of heavy paper on top of a smooth flat surface, such as a Formica topped counter.

slab is size and shape of finished piece

Step #2 Detailing clay positive

A variety of relief details are possible including undercuts and extremely fine details. Finer clays will give more detail while coarser, rougher clays yield less detail. This "waste mold" process requires no mold release coating.

no mold release coating is needed

undercuts are fine

Step #3 Build mold box

Trim away excess paper around the perimeter of the clay positive. Set up a mold box around the clay using 3/4" thick boards, on all four sides. Leave about 1" clearance between the edge of the clay and the mold box. The height of the mold box should be 1" above the highest point on the clay positive.

mold boards used around clay positive

Step #4 Apply splash coat

Face the clay positive with a thin creamy layer of plaster called a "Splash Coat". Pour a steady thin stream, approximately 1/16" to 1/8" thick, trying to flood it on evenly and not trap air. This coat provides a nice release surface and is soft so it pulls away easily from the glass keeping the fine details of the cast.

Splash Coat recipe (by weight):

1 part Pottery Plaster #1 1 part 200 mesh Silica add water to make a creamy consistency

"Splash Coat" acts as mold release

"Splash Coat" is soft and will keep the fine details of the cast

Step #5 Fill mold box with plaster

Use "Gerry's Mold Mix" to finish filling the mold volume. It is a stronger, coarser material than the Splash Coat. The casting mix should have the consistency of a heavy cream. It should flow into the mold box, not so runny that it flows under the box not so thick that it traps air.

over fill mold box slightly

When the plaster is starting to set, it is screed off the surface with a straight edge leaving a flat surface. This insures the top surface is parallel to the bottom. It is this flat surface that will be flipped over and placed onto the kiln shelf.

excess plaster screed off

plaster mold setting up undisturbed

Step #6 Remove clay

After the plaster has set, the mold box is removed.

carefully remove clay

Allow plaster to set at least 30 minutes before flipping over and removing clay. Save clay for weighing when calculating glass weight later. Wash mold gently to remove any clay residue, any small repairs or changes to the mold face can be done at this time.

Step #7 Dry empty plaster mold

Dry mold with or without forced air or heat for several days. This releases a large amount of water. After air-drying finish drying the mold upside down in your kiln. I ramp1 minute to 200F, hold at 200F for six hours, ramp to 600F over 12 hours then turn off the kiln and let it cool. The plaster mold should be as dry as possible. When the mold is really dry, it will be more stable, be less apt to warp during the glass cast firing and leave a cleaner glass surface. However because the molds are so dry, they have to be handled with care. Try not to handle them too much. If possible just turn them over and fill. 

dry plaster mold ready to fill with glass

Step #8 Filling plaster mold with glass

Billets

Hand cast billets, 6" x 9" x .75" glass "bricks", are a great way to get very clear castings in a range of colors. This form lends itself to casting large-scale tiles/ slabs/ panels very quickly. The colors are consistent across multiple molds and multiple firings.

System 96 Casting Billets

How to fill your mold with glass

Filling molds requires some calculating. It is necessary to determine the volume of glass needed to fill the mold volume. Weigh the clay removed from the mold then add 20%. This weight should be the corresponding glass weight needed. For simple square or regular shapes fill volume at the rate of 2.5 grams per cubic centimeter or 156 lb per cubic feet.

Once the amount of glass is known, place the glass in the mold covering the entire mold surface. If using billets or sheets to fill your mold, leave a slight gap between the glass and the mold walls. This will prevent touch points (that can be very sharp) from forming. Pile the remaining glass in a shallow mound highest in the center and also over deep cavities. If using sheet glass or billets, make sure the glass is clean! Dust and contaminates like fingerprints can show up in the cast and can contribute to devitrification. As the glass starts to melt it will flow down into the mold details and then flow out to the mold wall and level out. Mounding the glass in the center provides enough material to move toward the mold walls first and then to roll up the side, forming a nice smooth edge.

various frit sizes and colors added to mold cavities

added cane elements to mold

casting billets used to fill up the mold

Step #9 Firing Schedule

Firing glass and firing clay are different. Clay requires most if not all the artist's time and attention focused on the climb in temperature and the time spent at the top of the firing temperature range. Glass on the other hand is less concerned with what's happening as the temperature goes up, but is very concerned with the rate at which it is cooled. Slow and careful are the watchwords for successful casting. Below is a firing schedule for this project using billets. A digital controller is the best way to follow this schedule.

Uroboros offers firing schedules, in table format, downloadable from the "specifications" section of their website. Their schedules cover the use of sheet glass in addition to frit and billets when casting.

Stage A	Ramp from room temperature to 1000° in 4 hours. Mold is dry. This is probably too quick for large sheet pieces but works fine with billets.
Stage B	Hold and Soak at 1000° 1 hour Time spent here pays off later in the firing. Let the kiln and contents soak to even out the temperature. Keep in mind that a pyrometer is reading the air temperature in the free air of the kiln; the temperature in the center of the mold can lag behind that reading. The dry mold is acting as an insulation layer so it takes time for the heat to penetrate into the deepest areas of the mold.
Stage C	Ramp to 1000° to 1550° in 4 hours Slow enough so the billets slowly melt together and squeeze the air out as they also slowly flow into the relief and out to the mold walls.
Stage D	Cool to 1525° over 1 hour Bubble management. The glass is slowly getting stiffer so any bubbles are being held under the surface. Here the glass is being given the time to melt and level out. All kilns are different and these temperatures may need some adjustment. For example, 1550° in one kiln may read 1575° in another.
Stage E	Cool to 1000° as fast as possible, venting kiln if possible Quick cooling here minimizes surface devitrification (surface scum).
Stage F	Soak at 1000° 1 hour, stabilize temperature prior to annealing Again, let kiln and contents reach a temperature equilibrium.
Stage G	Anneal to 800° over 12 hours
Stage H	Cool to 500° over 6 hours, then turn off This cooling cycle is for my, slow cooling, fiber walled kilns. If you use a quick cooling brick kiln, add Stage I.
Stage I	Cool to 100° over 2 or more hours. Remember, the mold is quite a good insulator, so the glass is only cooling through the top face.

The work can safely be unloaded when the kiln is off and the glass is barely warm to the touch. Again, if the glass is large or thick or complex, waiting longer to unload is better than unloading too soon. Slow down. You risk breakage if the interior of your piece is significantly warmer to the touch than the surface, when glass is unloaded and the mold is broken away. If there is too much difference in the air temperature and the glass temperature, the glass will break. With large or thick work the kiln can be opened and several layers of newspaper laid down on the glass. This will give a tiny bit of insulation to the face while giving the glass still in the mold time to catch up, but it is not as safe as leaving the kiln closed longer.

fired glass mold coming out of the kiln close-up of fired glass in mold coming out of the kiln Step #10 Breaking the mold away from the glass This is the fun part. You finally get to see the glass! The molds made with "Gerry's Mold Mix" can be broken away by hand quite easily. Care should be taken with fine or fragile details. Running water and a stiff brush will scrub off remaining mold bits. Also a pressure washer works well to remove any mold bits, especially from rough surfaces or deep cavities. mold material carefully broken away from the glass casting detail of fired casting Here is the finished cast table!