Module 3: Assessment Quiz

Module: U12M3 - Hot Glass Techniques Passing Score: 70%

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What type of glass is preferred for flameworking (torch work)?

Borosilicate, because its low CTE resists thermal shock during rapid heating and cooling

Soda-lime, because it has a lower melting point and is easier to shape

Lead glass, because it flows smoothly at low temperatures

Fused silica, because it can withstand the highest temperatures

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Explanation: Borosilicate glass (CTE 3.3 x 10⁻⁶ /K) is preferred for flameworking because its low thermal expansion makes it resistant to the rapid, localized heating and cooling inherent in torch work. Soda-lime glass (CTE 9) would crack from thermal shock under the same conditions.

What is the typical full fuse temperature for soda-lime glass in a kiln?

1480-1530°F (804-832°C)

1100-1200°F (593-649°C)

1700-1800°F (927-982°C)

900-1000°F (482-538°C)

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Explanation: Full fuse temperature for soda-lime glass is typically 1480-1530°F. At this temperature, separate glass pieces merge completely into a single smooth piece. Lower temperatures produce tack fuse (pieces bond but retain individual shape), and higher temperatures risk devitrification or kiln shelf damage.

What is the difference between a full fuse and a tack fuse?

Full fuse merges pieces into one smooth layer; tack fuse bonds pieces while retaining their individual shapes and textures

Full fuse uses higher-quality glass; tack fuse uses recycled glass

Full fuse is done in a kiln; tack fuse is done with a torch

Full fuse requires annealing; tack fuse does not

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Explanation: Full fuse heats glass to the point where separate pieces completely merge into a single smooth surface. Tack fuse uses a lower temperature so pieces bond together at their contact points but retain their original shape, texture, and height — creating a dimensional surface.

What is glass slumping?

Heating fused glass over or into a mold so it softens and takes the mold's shape under gravity

Rapidly cooling glass to create a textured surface

Grinding glass into a curved shape using wet abrasives

Pressing hot glass between two flat plates to flatten it

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Explanation: Slumping heats already-fused glass to a temperature where it softens enough for gravity to pull it into (or over) a mold shape. Slumping temperature is lower than fusing temperature — the glass needs to be pliable but not fluid enough to lose detail or stick to the mold.

What is devitrification and what causes it?

Crystallization of the glass surface caused by holding at elevated temperatures too long or firing too slowly through critical ranges

Discoloration caused by incompatible glass types mixing during fusing

Air bubbles trapped between glass layers during kiln firing

Cracking caused by thermal shock when opening the kiln too early

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Explanation: Devitrification is the formation of crystals on the glass surface, appearing as a white, hazy, or scummy coating. It occurs when glass is held at temperatures in the devitrification range (typically 1100-1300°F for soda-lime) for too long, allowing atoms to rearrange from amorphous into crystalline structure.

What is the purpose of the annealing soak in a kiln firing schedule?

To hold at the annealing point long enough for internal thermal stress to relieve through atomic rearrangement

To allow the glass to reach its maximum temperature uniformly

To burn off contaminants from the glass surface

To allow kiln shelf primer to cure before the glass contacts it

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Explanation: The annealing soak holds the glass at its annealing point (approximately 960°F/516°C for soda-lime) for 30-60 minutes. At this temperature, glass atoms have enough mobility to rearrange and relieve internal stress caused by the heating cycle, but not enough energy to deform the piece.

Why must kiln shelves be coated with kiln wash (shelf primer) before fusing?

To prevent molten glass from permanently bonding to the kiln shelf surface

To add color to the bottom of the fused glass piece

To insulate the glass from the shelf's temperature

To prevent the kiln shelf from cracking during firing

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Explanation: At fusing temperatures, glass becomes sticky and will permanently bond to an uncoated kiln shelf. Kiln wash (a refractory coating, typically alumina hydrate or boron nitride) creates a release layer that prevents adhesion and allows the finished piece to be removed cleanly.

What is the critical cooling range in a soda-lime glass annealing schedule?

From the annealing point (~960°F) to the strain point (~900°F) — cooling must be very slow

From peak temperature to the annealing point — cooling must be very slow

From room temperature to 200°F — the first phase of heating

From the strain point to room temperature — the final cooling phase

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Explanation: The critical cooling range is between the annealing point and strain point. In this range, glass is transitioning from a slightly mobile state to a rigid state. Cooling too fast through this range locks in permanent thermal stress. Below the strain point, faster cooling is safe.

What fuel gas combinations are commonly used for flameworking in educational settings?

Propane/oxygen or natural gas/oxygen for borosilicate; propane/air for soft glass

Acetylene/oxygen only, similar to welding

Butane/air from standard lighter-style torches

Hydrogen/oxygen for all glass types

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Explanation: Educational flameworking typically uses propane or natural gas mixed with oxygen for borosilicate work (higher temperatures needed). Soft glass (soda-lime) can be worked with propane/air torches at lower temperatures. Acetylene produces too much carbon and is not standard for glass work.

What is the recommended maximum ramp rate when heating soda-lime glass to fusing temperature?

300-500°F per hour, depending on piece thickness and complexity

As fast as possible to prevent devitrification

50°F per hour to prevent any thermal stress

1000°F per hour for efficient production

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Explanation: Ramp rates of 300-500°F per hour are typical for soda-lime fusing. Faster rates risk thermal shock (especially for thick or complex pieces). Slower rates through the devitrification zone (1100-1300°F) actually increase devitrification risk — the key is to move through that zone steadily, not linger.

Why is it important to anneal flameworked beads even though borosilicate is thermal-shock resistant?

Even borosilicate develops internal stress from uneven heating during torch work that causes delayed failure if not annealed

Annealing changes the color of the glass to its final shade

Borosilicate is not actually thermal-shock resistant when worked by torch

Annealing hardens the surface to prevent scratching during wear

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Explanation: While borosilicate resists thermal shock from rapid temperature changes, flameworking creates localized hot and cold zones within the piece. These temperature differentials create internal stress. Without annealing, the bead may crack hours or days later when subjected to minor thermal or mechanical stress.

What is the purpose of a "bubble squeeze" segment in a kiln fusing schedule?

A hold at a temperature just below full fuse that allows trapped air between glass layers to escape before the glass seals

A rapid cooling step that forces air bubbles to the surface

A pre-heating step that removes moisture from the glass

A post-fuse hold that causes intentional bubble patterns for decoration

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Explanation: The bubble squeeze hold (typically 1350-1400°F for soda-lime) softens the glass enough for trapped air to escape at the edges before the pieces fully fuse and seal. Without this step, air trapped between glass layers forms permanent bubbles in the finished piece.