Bullseye Glass Mold #8453, Circle in Square 5.9"

The Bullseye Glass Mold 8453 is a slumping form that shapes glass into a circle centered within a square, producing a finished piece that combines both geometries in a single firing. A pre-cut glass sheet or fused blank is placed over the form, then fired on a controlled ramp so gravity draws the glass down onto the circle in square shape as it softens.

The mold measures 5.9 inches by 5.9 inches by 1.6 inches (15 x 15 x 4 cm) on the outside. These are outer dimensions for planning kiln fit, not the finished glass dimensions. The forms are handmade, so individual molds vary slightly in size; measure your specific mold before cutting glass. The form is slipcast from a specially formulated clay body by Creative Ceramics in Europe, then imported and sold by Bullseye Glass. The surface is exceptionally smooth, accepts kiln wash uniformly, and does not crack under repeated proper use.

Like all ceramic slumping forms, this mold is COE agnostic. It works with COE96, COE90, or other compatible fusible glass. Match your firing schedule to the glass and thickness you are working with.

What does the Circle in Square mold make?
It slumps a glass sheet or fused blank into a circular shape centered within a square format, giving the finished piece both a defined circular center and a clean square outer edge in one firing.

How does slumping work with this form?
A pre-cut glass blank is placed over the mold surface, then fired on a slow ramp. As the glass softens, gravity draws it onto the form so it takes the circle in square profile. After the slump is complete the glass is annealed and cooled slowly before the kiln is opened.

What size glass do I cut?
The outer mold measures 5.9 inches by 5.9 inches, but because these molds are handmade they vary slightly from piece to piece. Measure your specific mold before cutting glass so your blank fits the form you have.

What COE glass can I use?
Any COE. Ceramic slumping forms are COE agnostic, so the same mold works with COE96, COE90, or other compatible fusible glass. Use a firing schedule matched to the glass you are working with.

How do I prep the mold before slumping?
Coat the forming surface with a kiln wash or shelf primer and let it dry completely before placing glass. The slipcast surface accepts kiln wash uniformly. Re-coat as needed between firings.

Specifications

• SKU: 153138
• Manufacturer: Bullseye Glass (BG8453 / design #8453 / item 008453-MOLD-M-EACH; slipcast by Creative Ceramics in Europe, imported by Bullseye Glass)
• Makes: one circle in square slumped glass piece
• Mold outer size: 5.9 inches x 5.9 inches x 1.6 inches (15 x 15 x 4 cm)
• Note on size: molds are handmade and vary slightly; measure before cutting glass
• Material: slipcast ceramic kiln slumping form, reusable
• Glass: any COE (COE agnostic)

Working Notes

These are guidelines, not guarantees. Every kiln fires a little differently, so test a small piece first when the result matters and follow a schedule suited to your glass and kiln.

• Coat the forming surface with a kiln wash or shelf primer, such as ZYP Boron Nitride Mold Primer, and let it dry fully before placing glass on the mold.
• Place your pre-cut glass sheet or fused blank over the form and center it carefully before closing the kiln.
• This mold is COE agnostic. Use a slumping schedule matched to your glass, and adjust the anneal soak temperature for your COE: about 900 F for COE90 or about 950 F for COE96.
• Confirm the slump visually during firing when possible, and extend the hold at slump temperature if the glass has not fully settled onto the form.
• Let the kiln cool fully before opening and removing the finished piece. Re-coat the mold surface with kiln wash as needed between firings.

Bullseye Suggested Slumping Schedule

This is a suggested starting schedule from Bullseye, based on slumping 6mm pieces in a Paragon GL24. It is not a guarantee. Every kiln fires differently. Confirm the slump visually and adjust the schedule for your glass and kiln.

AFAP means as fast as possible, with no ramp rate limit. Segment 4 lets the kiln cool to room temperature naturally with the door closed. The 900 F anneal soak in segment 2 suits COE90; adjust to 950 F for COE96.

Helpful downloads: Suggested Slumping Schedules from Bullseye Glass (PDF)

Description updated: July 6, 2026

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