Sovereignty Module: Salt the Fire
Complete Soda and Salt Firing: From Kiln to Vapor Glaze
Soda and salt firing create glaze directly in the kiln by introducing sodium compounds that vaporize and coat pottery. This campaign covers the chemistry, kiln preparation, firing process, and surface effects.
Chapter 1: Vapor Glazing Basics
| Method | Material | Introduction | Temperature | Surface |
|---|---|---|---|---|
| Salt firing | Rock salt (NaCl) | Thrown into firebox | 2300-2400°F | Orange-peel texture |
| Soda firing | Soda ash (Na2CO3) | Sprayed as solution | 2200-2400°F | Smoother than salt |
| Soda firing | Baking soda (NaHCO3) | Sprayed or thrown | 2200-2400°F | Smooth, variable |
| Wood firing (natural) | Wood ash (contains sodium) | From fuel | 2200-2400°F | Subtle, natural |
Chemistry: 1) Sodium (Na) is a powerful flux. 2) At high temperature, sodium vaporizes and fills the kiln. 3) Sodium vapor reacts with silica and alumina in the clay body. 4) This reaction forms a thin layer of glass (glaze) on the pot surface. 5) The glaze is permanent and food-safe. 6) Every exposed surface in the kiln gets coated (including kiln walls and shelves).
Chapter 2: Salt Firing
Salt firing process: 1) Load kiln with unglazed pots (or pots with slip decoration). 2) Fire kiln to 2300°F (cone 10). 3) When temperature is reached, throw rock salt into firebox. 4) Salt amount: 1-2 pounds per cubic foot of kiln space. 5) Introduce salt in 3-4 additions (not all at once). 6) Wait 15-20 minutes between additions. 7) Salt vaporizes and fills kiln. 8) Check glaze development with draw rings (small test rings on wire). 9) Pull draw ring, examine surface. 10) When surface is glossy, stop adding salt. 11) Continue firing for 15-30 minutes (heat soak). 12) Cool kiln normally.
| Factor | Specification | Effect |
|---|---|---|
| Salt amount | 1-2 lbs per cubic foot | More salt = thicker glaze |
| Additions | 3-4 separate additions | Even distribution |
| Temperature | 2300-2400°F (cone 10-11) | Must be hot enough to vaporize |
| Atmosphere | Reduction preferred | Richer colors |
| Clay body | High silica content | Better glaze formation |
| Draw rings | Test pieces on wire | Monitor glaze development |
Chapter 3: Soda Firing
Soda firing advantages: 1) No hydrochloric acid fumes (salt firing produces HCl gas). 2) Smoother surface than salt. 3) More control over glaze thickness. 4) Can be sprayed for even distribution.
Soda solution: 1) Mix soda ash or baking soda with water. 2) Ratio: 1 pound soda per 1 quart water. 3) Add 1/4 cup soda feldspar (optional, improves melt). 4) Mix thoroughly (soda dissolves in hot water). 5) Spray into kiln through ports using garden sprayer. 6) Or mix with sawdust and throw into firebox. 7) Introduce in 3-4 additions at cone 10.
| Method | Soda Form | Introduction | Distribution | Control |
|---|---|---|---|---|
| Spray | Soda ash in water | Pump sprayer through ports | Even | Good |
| Throw (dry) | Baking soda | Thrown into firebox | Variable | Moderate |
| Throw (wet) | Soda + sawdust balls | Thrown into firebox | Variable | Moderate |
| Drip | Soda solution | Dripped onto coals | Gradual | Good |
Chapter 4: Surface Effects
| Effect | Cause | Appearance |
|---|---|---|
| Orange peel | Heavy salt/soda deposit | Textured, bumpy, glossy |
| Flashing | Light sodium vapor + iron in clay | Warm orange, peach, red |
| Gloss | Moderate sodium deposit | Smooth, glossy surface |
| Matte | Very light sodium deposit | Soft, dry surface |
| Color variation | Kiln atmosphere + clay chemistry | Variable colors across pot |
| Wadding marks | Wadding under pot | Unglazed circles on bottom |
Slip decoration under vapor glaze: 1) Apply contrasting slip to pot before firing. 2) Different slips react differently with sodium vapor. 3) White slip under soda: smooth, white-cream surface. 4) Iron-bearing slip under soda: rich brown-orange. 5) The combination of slip and vapor glaze creates complex surfaces.
Chapter 5: Kiln Preparation
| Preparation | Purpose | Method |
|---|---|---|
| Kiln wash (heavy) | Protect shelves from sodium glaze | 3-4 coats alumina/kaolin wash |
| Wadding | Prevent pots sticking to shelves | Alumina + kaolin + sand balls |
| Ports | Allow salt/soda introduction | Holes in kiln wall near firebox |
| Draw ring holes | Allow test piece extraction | Small holes in kiln door |
| Chimney damper | Control atmosphere | Adjustable damper plate |
Reference Card
- Sodium is the glazing agent (both salt and soda contain sodium; at high temperature, sodium vaporizes and reacts with clay to form a glass coating; this is the only glazing method that creates glaze inside the kiln). 2. Salt produces hydrochloric acid (when salt (NaCl) vaporizes, it releases hydrochloric acid gas; this is corrosive and toxic; salt kilns must be outdoors with good ventilation). 3. Soda is the safer alternative (soda ash and baking soda do not produce hydrochloric acid; soda firing is safer for the potter and the environment while producing similar results). 4. Everything in the kiln gets coated (sodium vapor coats every exposed surface, including kiln walls, shelves, and the backs of pots; protect shelves with heavy kiln wash and use wadding under pots). 5. Draw rings monitor glaze development (small clay rings on wire can be pulled from the kiln during firing to check glaze thickness; this is the only way to know when to stop adding sodium). 6. Clay body chemistry affects the result (clay bodies high in silica and alumina develop better vapor glaze; different clay bodies produce different colors and textures with the same firing). 7. Reduction enhances color (firing in a reducing atmosphere (restricted air) produces richer, more complex colors than oxidation; most soda and salt firers prefer reduction). 8. Vapor glazing connects to ancient tradition (salt glazing has been practiced since the 14th century in Germany; soda firing is a modern evolution; both methods create surfaces impossible to achieve any other way).