Testing standards are important to both the studio artist and his or her customers. While we may give specific instructions on the care, handling, and durability of our work, the truth is those tips can easily be forgotten and buyers expect our pots to act as well as commercial products. The responsibility falls to us to properly test our ware, accurately read the results, and ultimately sell a safe, durable product.

Standards Science Standards describe methods for testing specific attributes of fired ceramics. It is important to recognize that the standards are not laws, which define what results testing must produce for products to be sold. Standards simply explain how to do the testing. Standards exist most commonly for functional ware and architectural ceramics. They are intended to measure factors that indicate whether the work is safe to use and whether it will be durable in service. Standards exist for the very simple reason that without them it would be very difficult to compare the results of testing by different artists or companies. For example, if tile for an installation is described as having a particular strength as measured by a specific standard, say ASTM C1505-01(2007)—Standard Test Method for Determination of Breaking Strength of Ceramic Tiles by Three-Point Loading, then it is a simple matter to compare the reported strength to that of a specification for the strength measured using the same test. Who creates standards? There is an entire alphabet soup of standards organizations around the globe prescribing tests. For ceramic artists in the US and Europe, there are at least six such organizations. The American Society for Testing and Materials (ASTM) is the main one, along with the American National Standards Institute (ANSI). In Europe the British Standards Institution (BSI), the German Institute for Standardization (DIN), the International Organization for Standardization (ISO), and the Committee for European Standardization (CEN), all write ceramic standards. Standards define ways to measure important indicators of the quality of ceramic art. For example, whether a ceramic sculpture will survive outdoors unchanged by the weather is an element of quality. ASTM C1026-10—Standard Test Method for Measuring the Resistance of Ceramic Tile to Freeze-Thaw Cycling can predict whether a sculpture fired from a particular clay body will have that quality.

Professional (Certified) Testing Labs ASTM International (formerly the American Society for Testing and Materials)—Recognized as the leader in developing and testing standards for product quality and safety. Approximately 12,000 ASTM standards are used around the world. The majority of the standards are designed for use by industry, which employes specific and expensive equipment. Ceramic testing includes: thermal shock resistance of glazed tile, breaking strength, resistance to chemical substances, lead and cadmium leaching, resistance to freeze-thaw cycling, microwave safe reheating, and impact resistance of tableware. Orton Cone Manufacturing, Materials Testing & Research Center (MTRC)— A full service independent, nonprofit research and testing laboratory that specializes in measuring the behavior of refractory ceramic materials. Ceramic tests include: brick and tile structural testing, crazing resistance by thermal shock, chemical resistance, water absorption, porosity, and compression (crushing) strength tests. Cutlery & Allied Trades Research Association (CATRA)—Testing and technical consulting services for cutlery and cookware (among many other things). Dishwasher testing for ceramics. Professional Service Industries, Inc. (PSI)—Various testing services. Environmental Research, Inc.—Various testing services.

Testing In The Studio Knowing that your ware, and your glazes in particular, are chemically stable requires that you have answers to several key questions: Is the glaze mature? Is it within acceptable limit amounts? Is your kiln firing consistently and evenly for each firing and throughout the kiln? Is the liner glaze protective enough? Have all instances of shivering and dunting been eliminated? There are several tests that can be done in the studio requiring only simple equipment and materials. Potters should be aware that these tests are only an indication of possible ceramic instability and they are not technical enough to guarantee that ware is absolutely safe. Accurate testing and analysis is best left to a certified chemical lab.

RESULTS: The average absorption for a mature, well vitrified clay body, at any temperature, will be below 5%—porcelains absorb an average of 0–1%, stonewares will absorb 2–3%, and earthenwares will have 8–12% absorption.
Absorption testing is useful as an indicator of the maturity, or vitrification of fired clay bodies. Vessels made from adequately vitrified ware do not leak liquids, do not become unsafely hot during microwave heating due to excess internal water, or deteriorate if they absorb water and are placed in freeze/thaw conditions. TEST: Make a 6-inch-long x 2-inch-wide x -inch-thick tile and fire it to the recommended maturing temperature. Weigh the tile to the nearest 0.01 gram and record that as the dry weight. Boil the tile while fully submerged in distilled water for five hours and let it soak in the water and cool for another 24 hours. After soaking overnight, remove the tile, pat dry, and weigh it again. This is the wet weight. The wet weight minus the dry weight divided by the dry weight, with the result multiplied by 100, will give the percentage of absorption of the clay body at that temperature. Fire your clay body to several temperatures above and below the maturing temperature and weigh the tests using the same method to determine the range of your clay body.


Freeze-Thaw testing determines if your clay body is safe from deteriorating under freezing and thawing conditions. If water becomes trapped in the pores of an unvitrified clay body and that water freezes and expands, the tremendous force exerted by the ice against the ceramic object will cause it to crack, crumble, and chip the object. TEST: To determine if your clay body is safe from cracking under freezing and thawing conditions, find the C/B ratio. Take the dry weight of your fired clay body sample (remember to always wipe the surface before weighing). Record this weight as D. Next immerse this entire sample in water for 24 hours at room temperature (do not boil the water). Remove the sample from the water, pat dry and take the saturated weight. Record this weight as C. Now immerse the sample in the water again and boil the water. Leave the sample in boiling water for two hours. After two hours of boiling, remove and pat dry. Weigh the sample. Record the weight as B. The math is as follows:

RESULTS: A proper outdoor clay body will have a C/B ratio of less than 0.78 (approximately)—representing the room for expansion in the fired clay bodies’ pores. Absorption and freeze-thaw testing information courtesy of Cushing’s Handbook by Val M. Cushing.

Microwave testing is for ware used for heating and defrosting foods in a microwave. Ceramic wares absorb minimal amounts microwave energy and radiation in a non-uniform manner, which may cause thermal shock and possible cracking or explosions. Do not use lusters on ware intended for microwave use. The main consideration for recommending microwave use should be whether the clay is fully vitrified, and if so, it most likely will not be a problem in the microwave. If it isn’t vitrified, it will most likely overheat. TEST: Absorption Test (same as above). RESULTS: 2–3% is the maximum recommended absorption allowed for safe microwave oven use. Mature porcelain and stoneware (approximate moisture absorption level is 2–3%) are often safe but earthenware will almost always be a high risk unless it has been totally encased in an excellent glaze.

Leaching/Resistance to Acids/Glaze Stability is one of the most important tests you can do to understand the stability of your glazes. Potters should expect that all functional pots will eventually come in contact with acidic foods (i.e. vinegar, citrus juices, tomato sauce, coffee) at varying strengths and for varying lengths of time. Potters should also know that all glazes (glasses) are slowly dissolving, it’s just that stable glazes dissolve slower. Unstable glazes are the result of unbalanced formulations (i.e. recipes with 30% or more barium, almost no silica or alumina, or a high metal content) and they must be tested or avoided on food contact surfaces. Other factors influencing glaze stability include impurities, heating and cooling during firing cycles, and fuming from other materials in the kiln (either firing at the same time or in previous firings). TESTS: Submerge a glaze test half way into a glass container of vinegar (or fill a newly made glazed container half full of vinegar) and leave it for several days (at least three). A similar test can also be done with a lemon slice. If your glaze is white, add a small percentage of cobalt to a 100 gram test batch of the glaze and do the vinegar test.

RESULTS: Remove the test and allow it to air dry (do not pat dry) then compare the possible changes in color, gloss, and texture above and below the liquid line. Any change in the acid exposed area, even slight differences, indicates that the glaze is subject to leaching.

Dishwasher testing asses the resistance and suitability of products washed in a standard operating dishwasher. Ceramic wares can be minimally to severely affected by the automated dishwashing process and the alkaline effects of dishwashing detergents. Potters should be fully aware of the possible effects of corrosion, loss of gloss, and cloudiness among other failures. TEST #1: Take two identical glaze tests and put one on a shelf and one in the dishwasher for two months (or at least 30 cycles). If the glaze is unstable, a comparison of the two test tiles will show evidence of fading and a longer or accelerated test is needed. TEST #2: The accelerated test simulates the alkaline environment of the dishwasher. Make a 5% soda ash solution (50 grams of soda ash in 1 liter of water.) Put the solution in a stainless-steel pot (a double boiler works best.) Do not use aluminum. Bring the solution to a boil, then reduce the solution to a simmer, submerge a glazed test tile in the solution (make sure to have an identical test set aside to compare afterward), cover the pot, and continue to simmer for six hours. Check the water levels continuously.

RESULTS: Remove and rinse the tile. Look for differences in color, gloss, and texture. Moderately stable glazes show only slight changes in surface sheen and color. Unstable glazes show visible fading of both gloss and color with an etched surface. This test is only an general approximation of the dishwashing environment and not a substitute for an extended cycle test. Microwave, leaching, and dishwasher testing information courtesy of Mastering Cone 6 Glazes by John Hesselberth and Ron Roy. (

This article was excerpted from May 2012 issue of Ceramics Monthly, here.