Saggar Firing with Soluble Salts

 

Saggar firing is the method of creating confined atmospheres in a kiln within a container or saggar. Depending on the type of firing, the saggar can be made out of anything from the traditional refractory clay to newspaper. Originally, saggars were used to protect finishes from the debris flying around the firing chamber from the burning wood or coal fuel source.

In the ceramic saggar process, various organic and combustible materials can be added including copper wire, seaweed, grasses, flowers, sawdust, copper carbonate, and table salt. In 2018, I began experimenting with firing soluble salts, which I had previously used solely in foil saggar firing. I wanted to be able to fire the soluble salts to hotter temperatures and doing so in ceramic saggars would make that possible—foil begins to break down around 1400°F (760°C).

My saggar process primarily uses soluble salts to obtain certain colors, with the occasional addition of organic materials or wire for further effect. I fire to a temperature range of 1500–1750°F (850–954°C). Color results are very dependent on achieving precise temperatures. With this method, in the lower temperatures a calcium chloride wash will be yellow (1), but at a higher range it disappears; ferric nitrate is a darker rose color in the lower range and brightens to orange at higher temperatures; cobalt sulfate is a black/gray in the lower range and at the higher range is a brighter blue. Note: See the sidebar on page 36 for a list of chemicals and soluble salts and their expected color results.

1 A 5-inch-diameter test tile: calcium chloride is the yellow stripe on the left side.

2 Inspired by Turner, porcelain tile wired together to hold combustibles in a saggar.

The development of my current technique happened by chance when I was firing a saggar with salts and I accidentally changed my pyrometer from Fahrenheit to Celsius  without realizing it. The ceramic saggar broke and got much hotter than normal at 1830°F (1000°C). I was using ceramic tiles in the saggars as buffers to direct smoke marks onto a clay box form and to hold organic materials in place. The result of this misfiring was brighter colors on the tile buffers (2).

I began to limit the organic materials included in my firings to avoid excessive reduction and explored the use of salt crystals as surface decoration. I use very little sawdust, table salt mixed with copper carbonate, and include a few saturated cotton balls leftover from applying the salts. These can make some interesting flash marks or blushes on a pot’s surface. I have made hundreds of tests, varying the surface base color, trying various crystals on the surface, and varying the temperatures. In addition to creating a very controlled environment for a ceramic work, saggars also protect the kiln and retain the burn-off to a certain degree—too much sawdust in the saggar is not good and muddies the colors, while the dried cotton balls that I use to apply the liquid salts supply some surprise flashing because they are fully saturated. As a bonus, using them in the saggar reduces toxic waste in the environment.

Safety Measures

Safety precautions must be taken when using soluble salts. Arne Ase’s book, Watercolour on Porcelain: A Guide to the Use of Water Soluble Colourants,1 is dedicated to the use of soluble salts. It’s a treasure trove of information about metal salts and high-fire porcelain; however, little mention is made regarding toxicity of these chemicals and their proper handling and disposal. Most of these chemicals are irritants, toxic, or worse—carcinogens. There are excellent resources online for information on toxicology, uses, disposal, etc.2 Soluble salts’ toxicity varies but they can attack the mucous membrane in the nose and cause skin and eye irritation as well as absorption through the skin. When using soluble salts, cover your arms and wear fitted goggles, a vapor mask, and nitrate gloves (3). See the MSDS links in the sidebar and follow the recommended practices for each chemical.

Using Soluble Salts

I mix very small amounts of solutions using 6.5 grams of soluble salts and 1/8 cup of water. For my colors/salt mixture I broke down the amounts Ken Turner recommended in his foil saggar workshop. Some chemicals require more concentrated washes where others require less to achieve specific results. It’s a matter of personal preference, so it’s best to experiment and continuously test. The following are my recommended ratios of metal salts to hot water:

  • 100 grams salts in 1 pint water
  • 50 grams salts in 1 cup water
  • 25 grams salts in 1/2 cup water
  • 12.5 grams salts in 1/4 cup water
  • 6.25 grams salts in 1/8 cup water

These chemicals will dissolve metals, and some lose their potency after sitting in water, so it’s best to only mix the amount you plan to use in one firing and mix in a non-reactive container.

3 Safety gear: Eye goggles, vapor mask, nitrile gloves, and long sleeves.

4 An unfired surface with salt crystals dissolving on a base layer of cobalt sulphate.

5 Potassium dichromate base and a variety of salt granules.

6 Copper sulfate on top; cobalt sulfate on bottom; potassium phosphate over all.

The Process

My pieces are porcelain, coated with a ball-clay-based terra sigillata, and burnished with a soft sponge before being bisque fired to cone 08. The porosity of this low bisque temperature is necessary for the absorption of the chemicals in the higher temperature saggar firing. Using cotton balls saturated with mixed soluble salts and water, I rub the mixture onto the surface. The porous surface absorbs the material instantly. Avoid over wetting the cotton ball. After applying the solution to several pots, I place the cotton ball on a surface to dry. I save the used cotton balls and add them to the saggar (see 12). When the pieces are dry, I apply various salt crystals: sea salt, potassium chloride, bismuth subnitrate, potassium phosphate, copper sulfate, and two types of winter de-icer: calcium chloride and magnesium chloride (de-icer is available at home stores or online). I sprinkle these crystals onto the surface then spray them with water and left to soak overnight (4). The water allows the salts to dissolve into the surface. I use teething rings to hold the pots in position while applying crystals onto the sides.

When using saggars, there should be a few tablespoons of sawdust, combined with a 50/50 mix of table salt and copper carbonate in the bottom of the saggar. If I want a black bottom, I use grasses or more sawdust plus the used cotton balls. If I want bright colors, I use less. The pot rests on a pronged stilt and there is a space of 1 inch around the pot. These are fired to 1700°F, or higher than the previous firing.

The combinations are infinite. I have hundreds of test tiles combining layering or sprinkling granules (5). Some chemicals work best in foils fired to a lower temperature, in the 1300°F (704°C) range. Some work better in ceramic saggars, fired in the 1700°F (927°C) range. Others work well in both ranges, but with different results. Some of the more interesting combinations were the fluxing of potassium chloride (6) and potassium phosphate over cobalt sulfate (7). The latter seems to flux more and create a glossy to crinkly melt.

7 Potassium phosphate crystals and potassium chloride over cobalt sulfate.

8 Refiring cobalt sulfate on porcelain to remove cloudiness; first firing to 1300°F.

9 Results after the second firing of cobalt sulfate (see 8) to 1500°F (815°C).

10 Third firing of cobalt sulfate (see 8) to 1750°F (954°C).

Additional Firings and Finishes

After the pieces are fired, they are cleaned, rinsed, and allowed to dry overnight, then I seal them with paste wax (I use Trewax). Note: These pieces are not safe for use with food. I have also begun re-firing pieces that I feel have more potential, occasionally adding more layers of colors/salt mixture or more salt crystals.

On an example vessel, in the first firing to 1300°F (704°C) the cobalt sulfate was cloudy (8). In the second firing to 1500°F (815°C), the color began to brighten, but still retained some cloudy gray color (9). In the third firing to 1750°F (954°C), the dark spots were gone (10).

Another refiring example was a copper chloride piece with dark smoky spots (11). I put it in a saggar with used dried cotton balls, a little sawdust, copper carbonate, and table salt (12), then fired it. Opening the saggar showed a more solid coloration on the surface (13). The final piece developed colors of blushed pink, rusty spots with halos, and flashes from the cotton balls.

11 First firing of copper chloride without a saggar, fired to 1650°F (898°C).

12 Using the spent, dried cotton balls for flashing in the prepared saggar.

13 Vessel with copper chloride (see 11) after refiring in a saggar to 1750°F (954°C).

14 Fired vessel (see 11, 13), 6 1/2 in. (17 cm) in diameter, cleaned and waxed porcelain.

List of Soluble Salts

Examples of chemicals and their common characteristics when fire to a temperature range of 1500–1750°F (850–954°C). Review the MSDS information sheets online for all chemicals you use.

Bismuth Subnitrate, Bi₅H₉N₄O₂₂
Acts as a flux when applied over cobalt or copper and creates a brighter and sometimes glossy to crusty surface. MSDS: https://bit.ly/3eVrsJM

Calcium Chloride, CaCl₂
Used for melting ice and in producing cheese. In a wash in lower temperature firing ranges, it will be yellow. At higher temperature firing ranges if disappears. When sprinkled over a metallic salt, it will melt into the surface and bleach out the color, creating dots. MSDS: https://fscimage.fishersci.com/msds/03900.htm

Cobalt Chloride, CoCl₃
Results in a black-gray in the lower temperature firing ranges and dark blue in higher temperature firing ranges. MSDS: https://bit.ly/2P3AdHn

Cobalt Sulfate, CoSO₄
Results in a dark gray in the lower temperature firing range and brighter blue in the higher temperature firing range. MSDS: www.theceramicshop.com/sds/CobaltSulfate.pdf

Copper Chloride, CuCl
Results in a pale green in the lower temperature firing range and may need a higher concentration of salts in solution to get color. Works well

as a second layer of color. MSDS: https://fscimage.fishersci.com/msds/05625.htm

Copper Sulfate, CuSO₄·5H₂O
This goes pinkish and also works well in combination with other colors. MSDS: www.theceramicshop.com/sds/CopperSulfate.pdf

Ferric Chloride, FeCl₃
Results in a dark rose in lower temperature firings and brighter going orange in higher temperature firings. MSDS: www.waterguardinc.com/files/90276626.pdf

Iron Sulfate, FeO₄S
Provides warm peachy colors to orange in the higher temperature range. MSDS: https://bit.ly/3eQnemx

Magnesium Sulfate (Epsom salts), MgO₄S
I was hoping for this to influence cobalt, but it is very weak. Crystals used directly on a layer of colored chlorides or sulfates will create white bleached out dots. MSDS: www.theceramicshop.com/sds/MagnesiumSulfate.pdf

Potassium Chloride, KCl
Can be too powerful and create crusts that eat terra sigillatas. In a more diluted concentration, it can give some yellow colors if used sparingly. MSDS: https://bit.ly/3hxVUeu

Potassium Dichromate, Cr₂K₂O₇
Gives a strong green in all firing temperature ranges used. Caution: Toxic. MSDS: https://fscimage.fishersci.com/msds/19370.htm

Potassium Permanganate, KMnO₄
Used as a filtering chemical for swimming pools, it is very corrosive. Use in the lower temperature firing range for mauve. MSDS: https://bit.ly/2XvcXGS

Potassium Phosphate, K₃O₄P
In a solution this can flux copper and cobalt; if used heavily, it can be corrosive to terra sigillata and crusty. MSDS: https://bit.ly/3huDNpR

Sodium Chloride (Table Salt) NaCl
Bleaches out color of layers beneath. MSDS: https://fscimage.fishersci.com/msds/21105.htm

Sodium Dichromate, Cr₂Na₂O₇
Provides a more subtle green than potassium dichromate. MSDS: https://bit.ly/3hwlGjr

Soda Ash, CNa₂O₃
Use as a wash or sprinkled over colored salt layer for small white spots.

1. Ase, Arne. Watercolour on Porcelain: A Guide to the Use of Water Soluble Colourants. Norwegian University Press, 1989.
2. Resources for information on Classifications, Toxicology, Material Data Sheets: https://pubchem.ncbi.nlm.nih.gov, www.hse.gov.uk/reach/resources/reachsds.pdf, https://bit.ly/39UbazU.

Marcia Selsor is a professor emerita at Montana State University, where she taught for 25 years. She served on the technical staff of Ceramics Monthly, and the boards of Potters Council (now ICAN) and the National Council of Education of Ceramic Arts (NCECA). She was accepted as a member of the Northwest Designer Craftsmen. Her studio is at her home in Red Lodge, Montana. For more information, visit http://marciaselsorstudio.com, or find her on Facebook at Marcia Selsor Studio.

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