# How Salt and Soda Glazing Works

Unit: Alternative Firing Methods
Topic: Salt & Soda Firing
URL: https://claybook.studio/learn/how-salt-and-soda-glazing-works/

# The Glaze That Comes from the Air

Salt firing and soda firing are atmospheric glazing techniques. Instead of applying glaze to each pot individually, the glaze is introduced into the kiln atmosphere during firing and deposits itself on every surface inside the kiln simultaneously: pots, shelves, kiln walls, and all.

## Salt Firing

In traditional salt firing, common salt (sodium chloride) is shovelled or thrown directly into a gas or wood kiln at peak temperature (typically around cone 10-11).

At that temperature, the sodium vaporises and reacts with silica and alumina in the clay body surface, forming a thin, skin-tight sodium silicate glaze directly on the clay. The result is:

*   A distinctive **orange peel** texture: a fine, slightly dimpled surface that is unlike any brushed or dipped glaze.
*   Extremely tight fit between glaze and clay: the glaze is part of the clay body surface, not a separate layer.
*   Chlorine gas is a by-product of the reaction. Salt kilns must be fired outdoors or with heavy-duty fume extraction.

## Soda Firing

Soda firing uses sodium bicarbonate or sodium carbonate (soda ash) instead of salt. The surface effects are very similar (the same tight fit and organic texture), but:

*   No chlorine gas is produced, making soda firing safer and more environmentally acceptable.
*   Soda is typically introduced dissolved in water and sprayed into the kiln through ports.
*   The surface texture tends to be slightly smoother than traditional salt.

## Did You Know?

[Salt glaze pottery](https://en.wikipedia.org/wiki/Salt_glaze_pottery) has a rich history dating back to 15th-century Germany, where the technique was used to produce the distinctive Bartmann jugs of the Rhineland. The characteristic orange-peel texture of salt and soda glazing is a direct result of sodium vapour reacting with the silica in [stoneware](https://en.wikipedia.org/wiki/Stoneware) clay bodies at high temperature.

## Practical Decision: Salt or Soda?

Choose based on your studio constraints:
*   Pick **salt** if your setup supports strict fume control and you want classic strong orange-peel character.
*   Pick **soda** if you want similar atmospheric effects with safer emissions and finer control via spray introduction.

## Pro Tip

Run side-by-side test loads with identical clay and slips in both methods. Direct comparison will teach more than theory in one cycle.

## Check your understanding

### Question 1: How is the glaze applied in salt and soda firing?

- [ ] A. Each pot is dipped in a salt brine before loading the kiln
- [x] B. Salt or soda is introduced into the kiln atmosphere, where it vaporises and deposits on all surfaces
- [ ] C. Salt is dissolved in the clay body before throwing
- [ ] D. Glaze is brushed on and salt is added as a top coat after firing

Tip: In atmospheric glazing, the glaze is not applied to the pots directly. Instead, salt or soda is introduced into the kiln atmosphere at peak temperature, where it vaporises and deposits itself as glaze on every surface inside.

### Question 2: Why is soda firing often preferred over traditional salt firing today?

- [ ] A. Soda produces thicker glaze deposits for more durable surfaces
- [x] B. Soda firing produces no chlorine gas, making it safer and more environmentally acceptable
- [ ] C. Soda is cheaper than salt for large kiln loads
- [ ] D. They are identical: the preference is purely traditional

Tip: Salt firing produces chlorine gas as a by-product when the sodium chloride breaks down at peak temperature. Soda firing (using sodium bicarbonate or soda ash) produces the same surface results without the chlorine gas, making it safer and more environmentally acceptable.

### Question 3: If two studios want similar atmospheric effects but one has stricter emissions limits, which method is usually the better fit?

- [ ] A. Traditional salt firing, because sodium chloride is easier to source
- [x] B. Soda firing, because it avoids chlorine gas while giving similar effects
- [ ] C. Either method, because emissions are chemically identical
- [ ] D. Neither, atmospheric glazing is impossible under regulations

Tip: Soda firing is usually preferred where emissions control is stricter because it avoids chlorine gas while still delivering atmospheric sodium-glaze effects.