Slide 002: Kiln Fusing and Slumping¶

Slide Visual¶

Kiln Fusing and Slumping

Slide Overview¶

This slide covers kiln-based glass fusing and slumping -- the techniques of bonding glass layers at high temperature and shaping fused pieces over molds. Students learn fuse levels (tack fuse, full fuse, casting), kiln preparation, firing schedules with specific ramp rates and hold times, slumping mold types, and how to design a complete firing program from initial ramp to final cooling.

Instruction Notes¶

Kiln fusing bonds separate pieces of glass into a single piece using controlled heat. The technique is distinct from flameworking in that the entire piece heats uniformly in an enclosed kiln, allowing larger and more complex compositions that would be impossible with a torch. Understanding fuse levels is essential because the peak temperature determines the final appearance of the piece.

Fuse Levels¶

There are three primary fuse levels for soda-lime art glass (COE 90, such as Bullseye):

Tack Fuse (approximately 1350-1400F / 732-760C): Softens glass enough to bond pieces at their contact points while retaining their individual shape, texture, and height. Pieces maintain distinct edges and dimensional relief. The glass surface becomes slightly glossy (fire-polished) but does not flow. Used for: dimensional texture effects, attaching small elements to a base layer, preserving the shape of stringers and frit.

Full Fuse (approximately 1480-1530F / 804-832C): Merges all pieces into a single smooth layer. Individual pieces lose their distinct edges and the surface becomes flat and glossy. The 6mm rule applies: glass at full fuse temperature seeks a thickness of approximately 6mm (1/4") due to surface tension equilibrium. If less glass is present, it will contract and thicken (pulling inward from the edges); if more glass is present, it will spread outward. This is a critical design constraint -- a single layer of 3mm glass will shrink and round its edges; a double layer (6mm) will maintain its outline.

Casting (1550F+ / 843C+): Glass becomes fluid enough to flow into a mold cavity. Requires specialized casting molds (investment plaster, ceramic fiber, or stainless steel). Produces solid, thick glass forms. Casting requires extended annealing cycles due to the thickness of the resulting pieces.

Kiln Preparation Checklist¶

Kiln preparation is a required pre-flight checklist before every firing:

Kiln wash on shelves: Apply kiln wash (shelf primer) to all shelves. Standard formula: 50% alumina hydrate (Al2O33H2O) + 50% kaolin, mixed with water to cream consistency. Apply three thin crossed layers (horizontal, vertical, diagonal) with a brush. Allow to dry completely (overnight is best; minimum 4 hours). Kiln wash creates a refractory barrier that prevents molten glass from permanently bonding to the shelf. Without kiln wash, fused glass adheres to the shelf and both are ruined.*
Alternative release materials: Fiber paper (Bullseye ThinFire or similar) is an alternative that produces a textured bottom surface. Easier to apply but single-use and more expensive per firing. Fiber paper does not require drying time.
Kiln furniture placement: Posts and stilts position shelves at the correct height for even heat distribution. Leave at least 1" clearance between shelves and kiln walls/elements. The bottom shelf should be 1-2" above the kiln floor.
Element inspection: Check all heating elements for breaks, sagging, or corrosion before firing. A broken element causes uneven heating and can ruin an entire load.
Thermocouple verification: The kiln controller reads temperature from a thermocouple, but the thermocouple measures air temperature near the wall, not shelf temperature. Actual glass temperature can differ by 20-50F from the controller reading. Use witness cones or a secondary thermocouple at shelf level for critical firings.

Complete Fusing Firing Schedule¶

A typical schedule for a basic full fuse of standard-thickness (6mm / two layers of 3mm) soda-lime glass (Bullseye COE 90):

Segment	Name	Rate	Target	Hold	Purpose
1	Initial ramp	300F/hr	1000F	0	Slow heat to prevent thermal shock
2	Process ramp	400F/hr	1250F	0	Through softening range
3	Bubble squeeze	200F/hr	1350F	30 min	Allows trapped air to escape before glass seals
4	Rapid ramp	AFAP (full)	1490F	0	Minimize time in devitrification zone
5	Peak hold	--	1490F	10 min	Full fusion at target temperature
6	Crash cool	AFAP (vent/open)	960F	0	Rapid drop through devitrification zone
7	Annealing soak	--	960F	60 min	Stress relief at annealing point
8	Critical cool	100F/hr	800F	0	Slow cool through strain point
9	Final cool	200F/hr	100F	0	Safe to open kiln below 100F

Segment Notes: - Bubble squeeze (Segment 3): This is the most commonly omitted step, and its absence is the most common cause of bubble defects. At 1350F, the glass is soft enough that trapped air between layers can escape at the edges. Hold for 20-30 minutes. Without this hold, the glass seals at the edges before interior air escapes, trapping permanent bubbles. - Crash cool (Segment 6): After peak temperature, cool as fast as possible to 960F. This rapid transit through the devitrification zone (approximately 1100-1300F) minimizes the time for crystal formation. Devitrification appears as a white, hazy, or scummy surface. Methods: vent the lid 2-4 inches, or (for small kilns) briefly open the lid fully. NEVER crash cool below 960F -- below the annealing point, rapid cooling creates thermal stress. - Annealing (Segments 7-8): Covered in detail in Module 1, Slide 002. The annealing soak at 960F (for soda-lime) allows atomic rearrangement to relieve stress. Critical cooling at 100F/hour prevents new stress from forming.

Tack Fuse Schedule Variation¶

For tack fuse, segments 1-3 are identical. Change segment 4 target to 1370F, hold 10 minutes. All cooling segments remain the same -- annealing requirements do not change based on fuse level.

Slumping¶

Slumping is always performed as a separate firing after fusing. The fused piece (now a flat, fused panel) is placed on or in a mold coated with kiln wash, and the kiln is heated to slumping temperature.

Slumping temperature: approximately 1200-1250F (649-677C) for soda-lime. At this temperature, gravity softens the glass enough to conform to the mold shape over 15-30 minutes. Slumping temperature must be carefully controlled: - Too low: glass does not fully conform to the mold, leaving gaps and incomplete shape - Too high: glass sticks to the mold, surface quality degrades, and the piece may stretch or thin

Slumping schedule (simplified for a standard 6mm fused piece):

Segment	Rate	Target	Hold	Purpose
1	300F/hr	1000F	0	Prevent thermal shock
2	250F/hr	1225F	20 min	Slumping -- glass conforms to mold
3	AFAP	960F	30 min	Annealing soak
4	100F/hr	800F	0	Critical cool
5	200F/hr	100F	0	Final cool

Mold types: - Ceramic drop molds: Glass drapes into a concave form. Reusable, durable, wide variety of shapes available commercially ($15-80). - Stainless steel molds: Reusable, requires kiln wash, good heat transfer produces crisp shapes. More expensive ($30-150). - Fiber molds: Made from ceramic fiber board, carved or formed to shape. Disposable (1-3 uses), inexpensive to make, good for custom shapes. Does not require kiln wash. - Stoneware molds: Handmade ceramic molds, must be bisque-fired and kiln-washed. Good for custom, one-of-a-kind shapes.

Key Talking Points¶

Three fuse levels: tack fuse (1350-1400F), full fuse (1480-1530F), casting (1550F+)
The 6mm rule: glass at full fuse seeks 6mm thickness due to surface tension equilibrium
Kiln wash on shelves is mandatory -- prevents permanent glass-to-shelf bonding
Bubble squeeze hold (1350F, 20-30 min) allows trapped air to escape before sealing
Crash cool through the devitrification zone (1300-1100F) minimizes surface crystallization
NEVER crash cool below the annealing point (960F for soda-lime) -- creates thermal stress
Annealing soak at 960F for 30-60 minutes relieves thermal stress
Slumping is always a separate firing after fusing -- lower temperature, gravity-driven
Slumping molds must be coated with kiln wash; temperature must be precise to avoid sticking

Learning Objectives (Concept Check)¶

[ ] Can the student differentiate between tack fuse, full fuse, and casting temperatures and their visual results?
[ ] Can the student design a complete firing schedule with appropriate ramp rates, holds, and cooling?
[ ] Can the student explain why slumping is performed as a separate firing after fusing?
[ ] Can the student explain the purpose and timing of the bubble squeeze hold?
[ ] Can the student describe the 6mm rule and predict how a single-layer vs. double-layer piece will behave at full fuse?

Adaptations for Different Learning Styles¶

Visual Learners¶

Side-by-side photos of the same design at tack fuse, full fuse, and casting -- showing progressive loss of texture and dimensional detail
Firing schedule graph with temperature on Y-axis, time on X-axis, and colored zones for each segment (labeled with purpose)
Before/after photos of bubble defects (pieces fired without vs. with bubble squeeze hold)
Video time-lapse of kiln fusing through the kiln viewing port (if available)

Kinesthetic Learners¶

Kiln loading exercise: students prepare a shelf (apply kiln wash, position kiln furniture, load test pieces) under supervision
Firing schedule design exercise: given glass type, thickness, and desired fuse level, students create a complete firing schedule using blank forms
Slumping mold selection: students handle different mold types and plan which mold to use for a given project shape
Post-firing evaluation: students examine their fired pieces and compare results to their schedule choices

Auditory Learners¶

Walk through a complete firing verbally: "We're ramping at 300 degrees per hour... it's been 3 hours and we're at 1000F... now we speed up to 400 per hour..."
Discussion: "Your piece came out with bubbles trapped between layers. Walk me through your schedule -- where did you go wrong?"
Group exercise: students talk through troubleshooting a firing that produced devitrification

Reading/Writing Learners¶

Blank firing schedule forms (pre-printed with segment numbers, empty columns for rate/target/hold/purpose)
Written exercise: "Explain in your own words why the crash cool exists and why you must NOT crash cool below the annealing point"
Reference table of firing schedules for different glass types, thicknesses, and fuse levels

Standards and References¶

NFPA 86 - Standard for Ovens and Furnaces: - Applicable to kiln installations in educational settings - Specifies clearance requirements (18" minimum from combustible materials for top-loading kilns) - Emergency shutdown procedures and ventilation requirements

OSHA 29 CFR 1910.106 - Flammable Liquids (for gas-fired kilns): - If using gas-fired kilns, specifies fuel line safety and emergency shutoff requirements - Most educational kilns are electric, but this applies to gas-fired installations

ASTM C1048 - Standard Specification for Heat-Strengthened and Tempered Flat Glass: - While focused on architectural glass, the annealing and thermal processing principles are directly applicable - Provides reference for understanding how controlled heating and cooling affect glass stress state

Bullseye Glass Technical Notes (TipSheet Series): - Industry-standard reference for art glass firing schedules - TipSheet 1: Annealing Thick Slabs; TipSheet 5: Kiln Wash; TipSheet 7: Bubble Troubles - Freely available from Bullseye Glass Co. -- recommended as student reference material

Session Details¶

Time Allocation: 40 minutes (20 min lecture/demo + 20 min firing schedule design exercise)
Breakpoints for Discussion:
After fuse levels: "You want to preserve the texture of crushed frit on a pendant surface. Which fuse level?" (Answer: tack fuse -- full fuse would flatten and smooth the frit)
After 6mm rule: "You're fusing a single 3mm sheet with small pieces on top. What will the finished edges look like?" (Answer: pulled inward and rounded -- the base seeks 6mm, so it contracts)
After bubble squeeze: "Can you add a bubble squeeze hold after you've already peaked? Why not?" (Answer: no -- once the glass is sealed at peak temperature, bubbles are permanently trapped)
After slumping: "You want a deep bowl shape. Your first slump produced a shallow curve. What do you adjust?" (Answer: higher temperature or longer hold -- but be careful not to overslump)

Discussion Prompts¶

Schedule Design: "You're fusing a 12mm thick piece (four layers of 3mm glass). How does your schedule change from the standard 6mm schedule?" (Longer bubble squeeze, longer annealing soak, slower critical cool)
Devitrification: "Your piece came out of the kiln with a white haze on the surface. Your schedule looks correct. What else could cause devitrification?" (Dirty glass -- fingerprints, dust, old kiln wash contamination)
Production Planning: "You have 20 pieces to fuse and one kiln. How do you maximize kiln utilization without compromising quality?"
Creative Constraint: "The 6mm rule means your single-layer piece will shrink. A student sees this as a problem. How could you see it as a design feature?" (Use the contraction to create organic, rounded edge shapes intentionally)

Instructor Notes¶

The firing schedule design exercise is the most important learning activity in this slide -- students must be able to create schedules independently before operating the kiln unsupervised
Common student error: setting the peak temperature too high "just to be safe." Higher temperature does not produce a better fuse -- it causes spreading, thinning, and loss of design detail
Always verify kiln wash is completely dry before loading -- wet kiln wash produces steam that causes bubbles and can blow glass off the shelf
Teach students to label every firing with a kiln log entry: date, glass type, schedule used, pieces loaded, results. This log is invaluable for troubleshooting and reproducing successful firings
SAFETY CALLOUT: Never open a kiln above 200F. Even at 200F, the thermal shock of cold air can crack pieces. Never reach into a hot kiln for any reason. Use long kiln tongs or wait for the kiln to cool to room temperature.
The kiln exterior can exceed 300F during peak firing on some models -- label the kiln with "HOT SURFACE" signage and restrict access during firing

Common Misconceptions Corrected¶

Myth: "Higher temperature = better fuse." Reality: The correct temperature produces the desired fuse level. Going higher causes over-fusing (loss of texture, spreading, thinning) and increases devitrification risk.
Myth: "You can fuse and slump in the same firing." Reality: In some advanced techniques this is possible, but for students, these should always be separate firings. Combining them requires precise temperature control and mold design -- errors produce stuck, ruined pieces.
Myth: "Kiln wash is optional if you're careful." Reality: Glass at fusing temperature WILL bond to unprotected kiln shelves. There is no technique substitute for kiln wash or fiber paper. This is a 100%-of-the-time requirement.
Myth: "The kiln controller temperature is the glass temperature." Reality: The thermocouple measures air temperature near the kiln wall. Actual glass temperature can be 20-50F different. Use witness cones or a shelf-level thermocouple for critical firings.

Accommodations for Neurodiversity¶

ADHD Support¶

The firing schedule form provides structure: students fill in blanks rather than creating from scratch
Break the exercise into two parts: "Design the heating segments first (5 min), then the cooling segments (5 min)"
Kiln loading is a hands-on, physical activity that provides a break from the conceptual schedule design

Autism Spectrum Support¶

Firing schedules are inherently structured, rule-based, and precise -- this maps well to systematic thinking
Provide exact numbers for every parameter rather than ranges (e.g., "1490F" not "1480-1530F")
The kiln preparation checklist is sequential and exhaustive -- present it as a required procedure, not a guideline

Dyslexia Support¶

Firing schedule form uses color-coded segments matching the graph (red for heating, green for holds, blue for cooling)
Provide pre-filled example schedules that students can modify rather than creating from blank
Fuse level comparison uses photographs labeled with temperature, not text descriptions

Sensory Processing Support¶

Kiln viewing produces bright orange/red light and significant radiant heat -- didymium glasses and standing at arm's length from the peephole
Kilns produce a low hum from relays clicking on and off -- this may be distracting in an otherwise quiet studio
The heat radiating from a firing kiln raises ambient temperature significantly in small rooms -- ensure adequate room ventilation and cooling

Last Updated: 2026-03-19 Content Review: Q1 2026