Bubble Removal

Part of Glassmaking

Techniques for removing air bubbles from molten glass to produce clear, strong products.

Why This Matters

Bubbles are the most common defect in handmade glass. Every batch of raw ingredients traps air during melting, and chemical reactions between silica, flux, and stabilizer generate carbon dioxide and other gases. Left unchecked, these bubbles — called “seeds” when small and “blisters” when large — weaken the glass structurally, scatter light through optical pieces, and create leak points in vessels meant to hold liquids.

For a rebuilding civilization, bubble-free glass is not merely an aesthetic preference. Laboratory glassware with bubbles cracks under thermal stress. Window panes with trapped gas distort vision and fracture along bubble lines. Lenses for microscopes or telescopes become useless if seeds scatter the light path. The difference between glass that works and glass that fails often comes down to how well you removed the bubbles.

The good news is that glassmakers have solved this problem using simple techniques for millennia. High temperature, extended melting time, stirring, and chemical fining agents all work. Understanding the physics behind bubble behavior lets you combine these methods effectively even with primitive equipment.

How Bubbles Form

Bubbles enter glass through three mechanisms:

Mechanical Trapping

When you load raw batch materials (sand, soda ash, limestone) into a crucible, air fills the spaces between particles. As the batch melts from the edges inward, pockets of air become enclosed in viscous glass. Finer-ground ingredients trap less air — this is one reason to grind batch materials thoroughly before charging the furnace.

Chemical Decomposition

The chemical reactions that create glass release gases:

Reaction	Gas Released	Temperature
Soda ash (Na₂CO₃) decomposition	CO₂	850-900°C
Limestone (CaCO₃) decomposition	CO₂	700-900°C
Organic contaminants burning	CO₂, CO	300-600°C
Sulfate fining agents	SO₂	1,200-1,400°C
Moisture in batch	H₂O steam	100-300°C

These reactions are unavoidable — they are part of glass formation. The key is ensuring the gases escape before you remove the glass from the furnace.

Reboil

If glass that has already been fined (cleared of bubbles) is reheated or held at high temperature for too long, dissolved gases can come out of solution and form new bubbles. This is analogous to opening a carbonated drink. Prevent reboil by not overheating finished glass and by working at consistent temperatures.

Thermal Fining (High-Temperature Hold)

The simplest and most important bubble removal technique is holding the glass at high temperature for an extended period. This works because:

1. Higher temperature = lower viscosity. At 1,100°C, soda-lime glass has a viscosity where small bubbles rise only millimeters per hour. At 1,300-1,400°C, viscosity drops enough that bubbles rise to the surface and pop within minutes.

2. Buoyancy drives bubbles upward. A bubble in glass experiences the same buoyancy force as an air bubble in water, but the high viscosity of glass means it rises much more slowly. The relationship is described by Stokes’ Law — rising speed is proportional to the square of the bubble diameter and inversely proportional to viscosity.

Practical Procedure

1. Melt the batch at your normal working temperature (1,050-1,150°C for soda-lime glass) until all raw materials have dissolved. This takes 4-8 hours depending on batch size and furnace efficiency.

2. Raise temperature to 1,250-1,400°C. This is the “fining temperature.” You need your furnace at maximum output.

3. Hold for 2-4 hours at fining temperature. Do not disturb the melt. Bubbles will rise to the surface and burst.

4. Reduce temperature back to working temperature (1,050-1,100°C) over 30-60 minutes before gathering glass for blowing or casting.

Furnace Capacity

Not all primitive furnaces can reach 1,300°C. If your furnace maxes out at 1,100-1,200°C, you must rely more heavily on stirring, chemical fining, and extended hold times. A well-insulated furnace with forced-air bellows can reach fining temperatures.

Time vs. Temperature Tradeoffs

Fining Temperature	Required Hold Time	Fuel Cost
1,400°C	1-2 hours	Very high
1,300°C	2-4 hours	High
1,200°C	6-12 hours	Moderate
1,100°C	24+ hours	Lower per hour but more total

Higher temperatures are more fuel-efficient overall despite the higher burn rate, because the dramatically shorter hold time more than compensates.

Mechanical Stirring

Stirring serves two purposes: it breaks large bubbles into smaller ones (which rise faster in proportion to their collective surface area), and it brings deep-trapped bubbles closer to the surface where they can escape.

Stirring Method

1. Use a refractory stirring rod: A solid iron rod works but contaminates the glass with iron oxide (green tint). Better options are mullite or alumina rods, or a rod made from the same clay as your crucible, pre-fired to vitrification.

2. Stir slowly and steadily: Fast stirring entrains new air. Move the rod in slow circles, reaching the bottom of the crucible. Lift and reinsert occasionally to fold the glass.

3. Timing: Stir during the initial melt phase, not during fining. Stirring during fining can trap new air and undo your progress.

4. Duration: 10-15 minutes of stirring per session, repeated 2-3 times during the melting phase with 30-minute rests between.

Squeeze Test

After stirring, let the glass settle for 30 minutes, then gather a small blob on an iron rod. Pull it into a thin thread and hold it up to light. Visible bubbles mean more fining time is needed.

Chemical Fining Agents

Chemical fining agents are substances added to the batch that decompose at high temperature, releasing large gas bubbles. These large bubbles rise quickly through the glass and sweep up smaller bubbles as they go — like a net dragged through water collecting debris.

Common Fining Agents

Saltpeter (potassium nitrate, KNO₃)

• Add 1-2% by weight of total batch
• Decomposes at 400°C, releasing oxygen and nitrogen
• Also acts as an oxidizer, burning out organic contaminants
• Readily available: forms naturally on manure-rich soil, cave walls, and stable floors

Salt (sodium chloride, NaCl)

• Add 0.5-1% by weight
• At high temperatures, chlorine gas bubbles form and sweep through the melt
• The chlorine also volatilizes impurities
• Disadvantage: chlorine is corrosive to furnace linings

Wood ash (if not already used as flux)

• Contains carbon that burns out as CO₂
• The gas generation helps stir the melt from within
• Already present if you use wood ash as your soda source

Arsenic and antimony oxides

• Historically the most effective fining agents
• Both are toxic — handle with extreme caution
• Arsenic trioxide: 0.2-0.5% by weight
• Only use if you have the materials and can work safely outdoors with wind at your back

Toxicity

Arsenic and antimony compounds are poisonous. Fumes during melting are dangerous. Use only outdoors with adequate ventilation and wind blowing fumes away from workers. Saltpeter and salt are much safer alternatives and sufficient for most applications.

Combining Methods

The best results come from combining approaches:

1. Grind batch materials to fine powder (reduces mechanical trapping)
2. Add 1-2% saltpeter to the batch
3. Melt at working temperature with periodic stirring
4. Raise to fining temperature and hold for 2+ hours without disturbing
5. Cool to working temperature before gathering

Evaluating Glass Clarity

Visual Inspection

Gather a small amount of glass on an iron rod and pull it into a thin rod or flatten it into a disc. Hold against a light source or the sky:

• Seed count: Count visible bubbles per square centimeter. Fewer than 2-3 small seeds per cm² is acceptable for most applications. Zero visible seeds is the goal for optical work.
• Color: Should be consistent throughout. Streaks indicate incomplete mixing.
• Transparency: Hold printed text behind a thin disc of glass. You should be able to read the text if the glass is properly fined.

Ranking System

Grade	Seeds per cm²	Suitable For
Optical	0	Lenses, prisms, laboratory ware
Window	1-3 small	Window panes, decorative ware
Vessel	3-10	Bottles, jars, non-critical containers
Utility	10+	Beads, aggregate, non-structural uses

Troubleshooting Persistent Bubbles

Problem: Bubbles reappear after fining

• Cause: Reboil from overheating or contamination
• Fix: Lower your working temperature. Check for organic contamination in tools or batch materials.

Problem: Large blisters near the surface

• Cause: Chemical decomposition gases trapped by a surface skin
• Fix: The glass surface cooled and formed a skin before interior gases finished escaping. Maintain fining temperature longer. Do not let the surface cool.

Problem: Tiny seeds throughout, even after long fining

• Cause: Furnace cannot reach adequate fining temperature
• Fix: Improve furnace insulation, add forced-air supply, or switch to chemical fining agents. Reduce batch size so the furnace can heat the smaller volume to higher temperature.

Problem: Strings of bubbles in blown pieces

• Cause: Moisture on the blowpipe or water droplets falling into the gather
• Fix: Preheat the blowpipe tip before gathering. Keep the furnace area dry. Do not blow wet breath through the pipe — take a breath, hold, then blow steadily.