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Molten Design Glass Art Gallery Tim O'Niell - Artist
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  Hot Cast Glass   |   Kiln Cast Glass   |   Kiln Formed Glass  
While most people are somewhat aware of glass blowing, fewer are familiar with hot cast or kiln formed glass. In the images and descriptions below, glass artist Tim O'Niell demonstrates.
Click on any image to see a larger view.

Hot Cast Glass

Hot Cast Glass into Metal Molds

Sand core mold surrounded by steel perimeter moldIn this demonstration of hot casting, the glass is contained by a steel perimeter mold and a boat-shaped core made of resin sand - a binder used in the foundry industry. The core shapes and leaves its negative image in the glass. It ultimately disintegrates and does not remain part of the piece.

Filling the ladle from the furnaceWe work as a team, one of us ladling while the other opens and closes the furnace door, also trimming excess glass from the ladle with shears.

Ladling the molten glass Here, I ladle hot glass over the sand core.

Steel perimeter mold around glass-filled sand core mold The steel mold now contains the glass. The boat-shaped core is visible along the center axis. When this piece eventually comes out of the annealing oven, the sand core will fall away.

Steel perimeter mold is removedOnce the glass has cooled enough to hold its shape, we remove the perimeter metal mold and slide the hot glass piece onto a board. We then carefully load it into an annealing oven that has been preheated to 900° F.

The finished hot cast glass piece The newly made piece remains in the annealing oven for several days, while the temperature gradually decreases in a cycle designed to let the stress come out of the glass. By the time we remove it, there will be no residual stress in the glass.

Hot Glass - Sand Casting

Tim fills the ladle from the furnaceThis demonstration, originally done for Pilchuck Glass School 2004 Hauberg Fellows, illustrates the technique of hot casting glass into foundry sand (also called "olivine sand") which is mixed with a small amount of bentonite (a naturally occurring clay) and water. Noted wood carver Scott Jensen, working in the Native Northwest Coast Art tradition, has carved the original in wood.

Sand casting demonstration at Pilchuck Glass School We sift the sand mixture into a metal box to get a light, consistent mass that will take a good impression. Then we cleanly push the pattern into the sand, compressing the sand around it with our hands and carefully removing the pattern.

Tim fills the ladle from the furnaceOne of us will ladle molten glass from the furnace into the impression while another handles the furnace door and trims excess glass from the ladle with shears.

Hot glass is poured into the sand mold impressionThis image shows the impression blackened with soot from an acetylene torch, which has carbonized the impression to create a mold release. With this coating on the sand, we are ready to pour the hot glass.

Molten glass glows orange in the sand moldThe temperature of the glass we dip from the furnace is 2,200° F, and it cools rapidly once removed. It glows orange as we pour it, still glowing in the sand mold.

The hot glass pieces go into the annealing ovenWe watch the glass as it cools in the mold and, when we feel it has cooled enough to be moved without changing shape, we scoop it, along with a jacket of sand, onto a board. It's then carefully loaded into the annealing oven, which has been preheated to 900° F.

The finished sand casted pieceOur newly made artwork will spend the next several days in the annealing oven (longer for thicker pieces), while the temperature gradually decreases in a cycle designed to let the stress come out of the glass. By the time we remove it, there will be no residual stress.

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Kiln Cast Glass

Kiln Casting

Tim O'Niell's studio kilnKiln casting (similar but not identical to kiln forming) involves placing pieces of glass in a mold at room temperature, then heating them so they melt into and fill the mold. In this firing I have several stainless steel molds coated with kiln wash (the blue color), which prevents the glass from adhering to the mold.

Glass billets and cullets fill the steel moldsI stack glass up in the mold, either billets (large blocks) or cullet (small chunks). Billets give a clean casting with few bubbles; cullet traps air as it melts, giving rise to numerous tiny air bubbles in the finished piece. This image shows billets stacked up on the right, in the boat mold, and cullet stacked in the pyramid mold on the left.

After the annealing period the kiln is openedWhen I close the kiln lid I will program the control to slowly heat the kiln from room temperature to 1,550° F, over seven hours. The temperature will hold at 1,525° F for several hours, then rapidly cool (10 hours) to the annealing temperature of the glass, holding ("soaking") at this temperature to remove stress from the glass.

One of the finished kiln cast piecesThe kiln is then gradually brought down to room temperature, at which point we see how the firing went.

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Kiln Formed Glass

Kiln Forming

Tim O'Niell's studio kilnIn kiln formed, glass is positioned over a mold or form and placed in the kiln at room temperature, then slowly heated until first it changes shape ("slumping"), and then it bonds to itself ("fusing").

Curved molds shape the flat glass piecesThis photo shows my kiln's interior containing two curved, stainless-steel molds (the pink surfaces). Resting atop the molds is the glass that will be slumped and fused in the same firing. On each side, there are two sheets of plate glass, a top and a bottom, with decorative items sandwiched between them. (The pink color on the mold is kiln wash, a mold-release commonly used in kiln work.)

Glass is slumped over the curved molds and the inclusions are fused into the glassWhen everything is in place, the kiln lid is closed and the digital controller is programmed to heat up to slumping temperatures (1,150° F - 1,200° F), then fusing temperatures (1,400° F - 1,500° F), which are held for predetermined durations based on the effect desired. The kiln is then cooled rapidly to annealing temperature (in this case, 960° F).

The finished Kiln Formed PieceFrom this point, I cool the kiln more slowly to allow the strain to come out of the glass. Once the temperature is close to room temperature, the lid is opened and the result revealed. This piece was slumped in two sections, which were then glued together to make the vessel.

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