What is low fire pottery?
Low fire pottery refers to clay items that are fired at relatively low temperatures, usually between 06°C (1110°F) and 1100°C (2000°F). This allows the clay to become hard and durable while still being porous enough to absorb glazes. Common low fire clay bodies include earthenware, raku, and terra cotta. Low fire glazes are specially formulated to mature and become glassy at these lower kiln temperatures.
Why glaze low fire pottery?
There are several reasons to glaze low fire pottery:
- Glazes make the pottery non-porous, allowing it to hold liquids without leaking.
- Glazes improve the appearance of low fired clay by making the surface smooth and glossy.
- Glazes can decorate the pottery with colors and textures not possible with just the bare clay.
- Clear glazes strengthen low fire clay bodies and make them more durable and resistant to scratching.
- Glazes allow low fire pottery to be used with food safely since they seal the porous surface.
So in summary, glazing is an important finishing step that improves the functionality, durability, and aesthetics of low fire pottery. Unglazed low fire pottery tends to be rough, absorbent, and fragile over time.
What types of glazes can be used?
There are several major categories of glazes suitable for low fire pottery:
Low fire commercial glazes
These premixed glazes are specially formulated to mature and flow properly at lower kiln temperatures. They are available from most ceramic suppliers in a wide variety of colors and finishes. Common types include:
- Gloss glazes – shiny and smooth surface
- Matte glazes – dull, non-reflective surface
- Crystal glazes – contain crystals that form in the glaze as it fires
- Crawling glazes – intentionally crackle during firing
- Speckled glazes – contain added coloring oxides that create specks
Commercial low fire glazes are beginner-friendly and achieve consistent results. However, they offer limited options compared to blending your own.
Cone 04-06 glaze recipes
Intermediate and advanced potters often use glaze recipes designed to mature between cones 04 and 06. These can be found in books and online databases. Blending your own glazes allows endless customization options. But more testing is required to achieve the desired effects.
Earthenware glazes
These glazes contain a high percentage of lead, allowing them to fuse properly at lower temperatures under 1100°F. While often brightly colored, lead glazes require safety precautions so are less common today.
Raku glazes
Raku firing involves removing pottery from the kiln while still glowing hot. Raku glazes are specially formulated to crackle, bubble, and break during this rapid cooling process. Common raku glaze recipes include crackle, lustre, naked raku, and horsehair.
How should low fire glazes be applied?
Proper application is key to achieving an even glaze layer without drips or bare spots. Here are some tips:
- Clean greenware first to remove dirt and oils.
- Glaze consistency should be like heavy cream. Adjust thickness with water if needed.
- Pour glazes into a larger container for dipping or a small one for brushing.
- For dipping, grip foot or bottom and submerge vertically 4-5 inches into glaze.
- For brushing, use a soft bristle brush and apply in overlapping strokes.
- Apply 2-3 coats for good coverage, allowing drying in between.
- Wipe away drips before drying using a damp sponge.
- Fire according to glaze instructions, generally to cone 06 or lower.
Going slowly and applying multiple thin coats produces the best results. Ensure pieces are fully dry before firing to prevent glaze flaws like pinholes and cracking.
What if bare spots appear after firing?
If there are unglazed bare spots after the pottery is fired, the most likely causes are:
- Not enough glaze was applied – Try 3-4 coats next time.
- Glaze was applied too thickly in some areas – The puddles dried too quickly before the glaze could even out.
- Problems with glaze fit – The glaze overlaps itself or pulls away from the clay body as it fires.
- Pinholes from air bubbles – Carefully remove bubbles with a pin between glaze coats.
- Greenware was dusty – Wipe away any dirt before glazing.
- Fired too quickly – Slow bisque firing allows glaze to adhere better.
To fix bare spots, wait for the kiln to fully cool down. Then carefully clean those areas and reapply the glaze using the same number of coats. Fire the piece again using the original glaze firing schedule. Small touched up areas may appear slightly different but are preferable to bare clay.
What causes glaze defects like crawling, shivering, and crazing?
Several common glaze flaws can occur while firing low fire pottery:
Crawling
The glaze pulls away from the clay body creating bare patches. Cause is poor fit between the glaze and clay. To prevent crawling:
- Use a glaze formulated for the clay body
- Apply thinner and more uniform glaze coats
- Fire at the temperature recommended for the glaze
- For homemade glazes, adjust the recipe or substitue ingredients
Shivering
Tiny flakes of the glaze come off leaving rough spots. This happens when the clay body shrinks during firing more than the glaze. To prevent shivering:
- Use a glaze with a lower thermal expansion
- Fire at a higher temperature so the glaze and body shrink similarly
- Compress clay better when throwing to reduce later shrinkage
Crazing
A network of fine cracks forms in the glaze surface. Crazing is caused when the glaze shrinks more than the clay body during cooling. To prevent crazing:
- Select a glaze with a higher thermal expansion
- Fire at a lower temperature
- Compress glazed pieces slowly in the kiln during cooling
Testing glaze and clay combinations plus adjusting firing schedules helps minimize glaze defects. But some characteristics like crazing may be desired for certain aesthetic effects.
How should low fire glazes be mixed?
For commercial premixed glazes, simply stir well before use to evenly distribute any settled colourants or ingredients. However, mixing custom glaze recipes from raw materials requires more care:
- Weigh out ingredients accurately using a gram scale.
- Mix dry ingredients first until uniform.
- Add liquids like water in small amounts while mixing.
- Mix for 3-5 minutes until smooth consistency with no lumps.
- Strain the glaze through an 80-100 mesh sieve to remove lumps.
- Test glaze fit on test tiles before applying to pottery.
Important mixing tips:
- Measure glaze materials by weight, not volume which can vary.
- Avoid inhaling dry glaze materials – wear a respirator mask.
- Only mix as much glaze as you will use up within 6 months.
- Remix or stir glazes before reusing older buckets that have settled.
Taking notes on custom glaze mixes and testing helps recreate results. Controlling glaze materials, mixing, application, and firing is key to reducing glaze defects.
What materials are used in low fire glazes?
While some natural materials like ash or clay can be used, most modern low fire glazes rely on refined ceramic chemistry materials. Some key ingredients and their roles include:
| Material | Functions |
|---|---|
| Frit | Lowers maturation temperature, promotes glass formation and flow |
| Silica | Source of glass former silica dioxide |
| Feldspar and nepheline syenite | Glass former, supplies alumina and alkalies |
| Kaolin | Raises glaze melt fluidity and increases durability |
| Talc | Source of magnesia and silica to improve glaze fit |
| Zinc oxide | Helps glaze melt and flow at lower temperatures |
| Bentonite | Improves glaze application, suspension, and adhesion |
| Colorants and opacifiers | Add aesthetic effects and opacity |
These base materials are combined in different ratios to create low fire glazes with the desired physical and visual properties.
What safety precautions should be taken?
When using hazardous glaze materials and firing kilns, proper safety gear and precautions are a must:
- Use a N-95 respirator when handling dry glaze ingredients.
- Wear a mask, apron, and gloves when mixing glazes.
- Clean up all spills right away to prevent skin and lung exposure.
- Wash hands thoroughly after any glaze contact.
- Use spray bottles instead of powder application to minimize dust.
- Ventilate your workspace when firing kilns to prevent toxic gas buildup.
- Keep a fire extinguisher and first aid kit on hand.
- Store all hazardous chemicals correctly in locked cabinets.
- Label all glazes mixes with application notes and firing schedules.
Never eat or drink around glaze materials. Take the time to follow basic studio safety habits to avoid accidental exposure or ingestion of harmful substances through skin contact, inhalation, or contamination.
Can low fire glazes be used in foodware?
Whether low fire glazed pottery is food-safe depends on the specific clay, glaze ingredients, and firing method used. There are some general guidelines:
- Lead glazes should never contact food as lead is toxic.
- Colored or metallic glazes may contain heavy metals like cadmium or copper.
- Unglazed terra cotta can absorb bacteria and may leach iron.
- Glazes fired to cone 06 or higher are usually inert and non-toxic.
- Low-porosity clay fired above 2200°F is safer for food use.
- Commercial glazes marked food-safe undergo toxicity testing.
- Plain, undecorated glazes in neutrals colors are simplest and safest.
When in doubt, either test your glazes or stick to known food-safe products only. Never use pottery with cracks, chips, crazing, or other defects around food.
Conclusion
Glazing transforms low fire pottery into a functional, beautiful, and durable finished product. With the right combination of clay, glaze, and firing technique, even beginner potters can achieve professional-looking results safely. Mastering application techniques plus learning the science behind mixing and firing glazes opens up creative possibilities with low fire ware. Always follow safety guidelines when exploring new glazes.