Techno File: Water

Water won’t dissolve everything, but it will dissolve more things than any other known liquid. This sometimes makes the most common ingredient in every clay and glaze a problem. It also raises the question, what’s the best water to use when mixing and forming?


Deflocculation: A reduction or elimination of the formation of loose clusters of tiny particles of clay and/or bentonite in a glaze, decorating slip, or casting slip. Deflocculation causes the slurry or slip to be very fluid while containing a low percentage of water.

Distilled Water: A type of purified water that has been boiled into vapor and condensed back into a liquid in a separate container. Impurities in the original water that do not boil below or near the boiling point of water remain in the original container.

Deionized (DI) Water: Water that has been treated to remove all ions—typically, this means all of the dissolved mineral salts.

Flocculation: The formation of loose clusters of tiny particles of clay and/or bentonite in a glaze, decorating slip, or casting slip. Flocculation permits suspension of coarse particles such as quartz, feldspar, and frit in a slurry that is 50% water by weight.

Hardness: With reference to water, hardness is the amount of calcium and magnesium dissolved in the water, typically given as milligrams per liter (mg/L) or parts per million (ppm), which is the same amount.

Reverse Osmosis (RO): Process that removes foreign contaminants, solid substances, large molecules, and minerals from water by using pressure to push it through specialized membranes. This system improves water for drinking, cooking, and other important uses.

Short Clay: A plastic clay body that is difficult to form because it cracks or breaks while it is being worked.

Hard Vs. Soft

Artists ask, what’s the best water to use in my studio—tap water that’s been softened, tap water that’s hard, tap water that’s naturally soft, distilled water, reverse osmosis (RO) water, deionized (DI) water, or even rainwater? The answer is tap water. Water from any of the other sources listed above is also suitable for ceramic-art processes, but it may cost you more and complicate your work in the process.

Many artists worry about hard water creating problems, and they are technically correct. Hardness in water can be problematic, but only in the extreme. In reality, most of us don’t have water that’s very hard. Here’s why. Hard water contains dissolved calcium (Ca) and magnesium (Mg). These two elements (and possibly others), are dissolved from rocks as water flows through the ground. The elements exist in solution in water as charged particles called ions. Hardness of water, by definition, is the quantity of Ca and Mg dissolved in it, often expressed in parts per million (ppm) or milligrams per liter (mg/L). Water with less than 60 ppm hardness or less is classified as soft. Water with a hardness of 61 or higher is classified as hard, and is most likely being treated with a water softener before it comes out of your tap.

Map of hardness of water in the US, showing the concentration of hardness as calcium carbonate in milligrams per liter. Colors represent stream flow from the hydrologic-unit area. Note: For a specific location or user, actual analysis of a sample or information from the water supplier is necessary to know the hardness of one’s water supply—municipalities and water utilities analyze the water they supply. Hardness analyses are available to customers. Photo: Courtesy of the U.S. Geological Survey.

For example, my water supply has a hardness of about 350 ppm. That’s classified as very hard and will shorten the life of my water-heater elements and plumbing pipes if it isn’t softened by exchanging the Ca and Mg with sodium (Na) in my water softener. The result? My tap water is classified as soft even if my water supply is hard. That’s common for studios where water is hard. Unless raw water bypasses the softener, which is sometimes done with the cold water, tap water is not hard enough to worry about in the studio. The only problem an excess of cations can cause is coagulation of slips or loss of plasticity in clay bodies.


Most studios from northern Maine to Houston, Texas, within 300 miles or so of the coast (except southern Florida) and in much of the Northwest receive tap water that is naturally soft. Parts of Minnesota, almost all of the arid Southwest, the Central Valley of California, and the east side of the Rockies out onto the Great Plains get water from aquifers with hard water. Treated water from lakes and rivers may be locally softer. Wells over limestone formations yield water that’s harder.

The U.S. Geological Survey classification for soft and hard water.

Does Purity Matter?

For some ceramic artists, the use of distilled water and the like seems logical. It’s really pure, right? Yes, but it’s also slightly acidic. That’s because, like all water, it absorbs carbon dioxide (CO2) from the air to form carbonic acid. Distilled water has a pH of about 5.8. A pH of 7.0 is neutral. In as little as two hours, distilled water will dissolve small amounts of soluble materials in clay or glazes. This process is the water getting back to the hardness it was before an industrial process purified it.

RO water has been treated by reverse osmosis filtration to remove dissolved solids. DI water has been treated differently, but for the same reason. Rainwater has been evaporated by nature, so it’s like distilled water, but it also picks up dust, organic matter like pollen, bits of algae, and mold spores as it falls through the air. All of these types of water cost money or effort to collect, are slightly acidic, and are not as effective in most ceramic processes as plain tap water.

If you want to use rainwater, you should let the solids settle out first. Then pour off the clear liquid and treat it with enough biocide such as bleach (¼ teaspoon per gallon of rainwater) to kill the algae and mold. You do not want mold growing in your clay or glazes. Clay mixed with water is naturally flocculated. Dispersants allow mixing dense, fluid casting slips containing clay by deflocculating the slurry of clay in water. An excess of cations from hard water may coagulate such slips, making them viscous and therefore difficult to cast.

Chlorine added to water by a city or other water management system may not be something everyone wants in their water. However, it will not cause problems with clay, slips, or glazes in the studio.

Simple Tests

A test of the suspension of a simple, clear glaze recipe:

shows that soft water, hard water, and distilled water all suspend the glaze the same at both 44 and 50% water by weight of the total recipe. An increase of the hardness of the distilled water sample in steps, from 500 ppm to 1770 ppm shows a very, very slight reduction in the rate of settling of the glaze ingredients. Using clay in glaze recipes at the rate of a minimum of 10% of the weight of the dry ingredients solves most suspension issues.

You can use commercially available test kits to measure your studio’s water hardness. The simplest use paper strips similar to pH paper. Dip a strip in a water sample, then compare its color to colors on a chart supplied with the strips. These are typically limited to some maximum hardness. I have seen strips that measure to only 200 ppm hardness, others to 425 ppm or even one that goes to 1000 ppm. The latter are likely not as precise on the very low end of the scale, and the former, of course, are of no use with high hardness water.

the author Dave Finkelnburg is a studio potter and practicing engineer. He earned his master’s degree in ceramic engineering from Alfred University.


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