Student Material 001: Hot Glass Techniques Reference Guide

Material ID: U12M3-MAT-001

Kiln Fusing Temperature Reference (Soda-Lime Glass, COE 90/96)

Fuse Level	Temperature Range	Result	Use Case
Slump	1200-1250°F (649-677°C)	Glass softens and drapes into/over mold	Shaping fused pieces into bowls, plates
Tack Fuse	1350-1400°F (732-760°C)	Pieces bond but retain individual shape	Dimensional texture, raised elements
Full Fuse	1480-1530°F (804-832°C)	Pieces merge into one smooth layer	Flat tiles, coasters, panels
Casting	1550°F+ (843°C+)	Glass becomes fluid, flows into mold	Sculptural forms, thick pieces

Standard Full Fuse Firing Schedule (Soda-Lime, 6mm)

Segment	Name	Rate	Target	Hold	Purpose
1	Initial Ramp	300°F/hr	1000°F	0 min	Gentle heating — prevent thermal shock
2	Process Ramp	400°F/hr	1250°F	0 min	Through softening range
3	Bubble Squeeze	400°F/hr	1375°F	20 min	Release trapped air between layers
4	Fire to Peak	AFAP	1490°F	10 min	Full fuse — pieces merge
5	Crash Cool	AFAP	960°F	30 min	Skip devit zone; anneal soak
6	Critical Cool	100°F/hr	800°F	0 min	Slow cool through stress zone
7	Final Cool	300°F/hr	100°F	0 min	Cool to room temp

Total estimated time: 8-10 hours

Standard Slump Firing Schedule (Soda-Lime, after fusing)

Segment	Name	Rate	Target	Hold	Purpose
1	Initial Ramp	300°F/hr	1000°F	0 min	Gentle heating
2	Slump Ramp	250°F/hr	1225°F	10 min	Glass softens into mold
3	Crash Cool	AFAP	960°F	30 min	Skip devit zone; anneal soak
4	Critical Cool	100°F/hr	800°F	0 min	Stress relief cooling
5	Final Cool	300°F/hr	100°F	0 min	Cool to room temp

Annealing Reference by Glass Thickness

Thickness	Soak Time at Annealing Point	Critical Cool Rate	Total Annealing Time
6mm (1/4")	30 min	100°F/hr	~3 hours
12mm (1/2")	60 min	50°F/hr	~6 hours
19mm (3/4")	90 min	35°F/hr	~10 hours
25mm (1")	120 min	25°F/hr	~16 hours

Rule of thumb: Doubling thickness approximately quadruples annealing time.

Annealing Points by Glass Type

Glass Type	Annealing Point	Strain Point	Critical Range
Soda-lime (COE 90)	960°F (516°C)	900°F (482°C)	960°F → 900°F
Borosilicate (COE 33)	1050°F (566°C)	950°F (510°C)	1050°F → 950°F

Flameworking Reference (Borosilicate)

Torch Flame Types

Flame Type	Appearance	Sound	Use
Oxidizing	Sharp inner cone, pointed	Hissing	High heat work, but can cause boiling
Neutral	Even cone, moderate	Moderate hiss	Standard working flame
Reducing	Bushy, yellow tips, soft	Quiet	Color striking, gentler heat

Torch Lighting Sequence

1. Open fuel valve (small amount)
2. Ignite with striker
3. Adjust fuel to small, stable flame
4. Gradually add oxygen to achieve neutral flame

Torch Shutdown Sequence

1. Close oxygen valve first
2. Close fuel valve second
3. Allow torch tip to cool — do not touch for 5 minutes

Glass Introduction Technique

1. Hold glass rod at 45-degree angle
2. Start at flame edge (cooler outer zone)
3. Slowly advance into hot zone over 15-20 seconds
4. Begin rotating when glass starts to glow
5. If tip pops off: you introduced too fast — thermal shock

The 6mm Rule

Glass at full fuse temperature seeks a natural thickness of approximately 6mm due to surface tension equilibrium.

Glass Amount	Behavior at Full Fuse
Less than 6mm total	Contracts inward and thickens to reach 6mm
Exactly 6mm	Stays in place, smooth surface
More than 6mm	Spreads outward to reach 6mm thickness

Design implication: Two layers of 3mm glass = 6mm total = stable at full fuse. One layer of 3mm = will contract. Three layers = will spread.

Common Failures Troubleshooting

Failure	Appearance	Cause	Fix
Devitrification	White haze/scummy surface	Too long in 1100-1300°F range	Ramp faster through devit zone; crash cool after peak
Thermal shock	Clean cracks from edges/corners	Ramp too fast or kiln opened too early	Slower initial ramp; never open kiln above 200°F
Incomplete fuse	Visible seams, unfused edges	Peak temp too low or hold too short	Increase peak temp 10-20°F or extend hold time
Bubbles	Air pockets between layers	No bubble squeeze step	Add 20-min hold at 1375°F before peak
Shelf adhesion	Glass stuck to kiln shelf	No kiln wash or kiln wash worn off	Recoat shelf; use fiber paper backup
Stress fracture	Cracks appearing hours/days later	Insufficient annealing	Longer soak, slower critical cool rate
Spreading	Piece flattened and expanded	Too much glass (>6mm) at full fuse	Reduce layers or lower peak temperature

Kiln Preparation Checklist

• [ ] Inspect kiln elements for damage
• [ ] Check thermocouple position (should be at shelf level)
• [ ] Inspect kiln shelf for cracks or old glass residue
• [ ] Apply kiln wash (3 thin crossed layers, dry between) OR place fiber paper
• [ ] Position shelf on posts at correct height
• [ ] Clean all glass with isopropyl alcohol
• [ ] Verify all glass is compatible (same COE)
• [ ] Arrange pieces with 1-inch spacing
• [ ] Program firing schedule into controller
• [ ] Record schedule in kiln log

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Last Updated: 2026-03-19