December 16, 2009
Any failure in a clay body or glaze represents hours of lost time and effort. A defective ceramic piece is not only lost, but when a replacement is needed, duplicate forming, glazing, and firing operations will be required, along with the extra fuel and material expenses. Furthermore, the kiln space used for the replacement could have been utilized to produce other pieces.
Understanding the cause and correction of any defect can, in many instances, prevent future flaws, which translates into increasing productivity. Unlike other defects that occur in clay body and glazes, the possible causes of lime pop are few. Therefore, tracking the source of the problem and taking the appropriate action will insure a fast correction.
Many potters have experienced a semi-elliptical 1/8 to 1/2 in. crack in their low-temperature bisque or high-temperature fired ware. Upon peeling back the defect, a conical hole reveals either a black or white nodule at the bottom. Lime pop occurs when moisture in the air comes into contact with a carbonized lime nodule, causing its expansion in an unyielding fired clay body. Unfortunately, this defect can occur when the unglazed or glazed pottery is removed from the kiln. It can also happen years later as the lime expands in the form of calcium hydroxide, which is always present and acting on the ware. In some types of building brick, lime pop can also be observed near the clay surface due to the same expansion reaction when in contact with moisture.
Interestingly, if lime is present in the clay body as a powder, the forces of expansion are not sufficient to crack the clay. In low-fire white clay bodies, powdered limestone (composed of more than 80% calcium or magnesium carbonate) is often added to prevent glaze crazing (a fine network of lines in the fired glaze).(1) When used in earthenware glazes, large percentages can cause crystal growth. In high-temperature glazes, limestone in powder form acts as a flux, bringing other glaze materials into a melt.(2)
Sources of LimestoneLimestone contamination in moist clay comes most frequently from plaster wedging tables or plaster bats, since plaster is composed of lime, gypsum and water. If the correct ratio of plaster to water is not used, the plaster cannot achieve its maximum strength when setting up. Soft or brittle plaster nodules of greater than 1/2 mm can enter the clay body in wedging or the reprocessing of scrap clay. Eventually, any plaster will degrade, causing the adhesive action of the moist clay to grab particles from the weakened plaster surface. To counter this type of mishap, a canvas cloth should be stapled on top of the plaster wedging board. Plaster bats should also be carefully inspected for any soft spots or concave areas, which could indicate past introduction of chips into the moist clay.
Raw clay can also be a potential source of lime particles. Limestone nodules can sometimes be found embedded directly in the seam of clay. Some clays form next to strata of limestone sedimentary rock and shells. If the mining operation does not carefully excavate the clay during removal, contamination can occur. As the material is excavated, limestone rocks can accompany the clay to the milling operation.
Contamination can also be introduced into clay, since limestone is frequently used in roadbeds where trucks bring the clay for stockpiling and processing. Air-floated clays, where the clay has been separated by a stream of air into different particle sizes, can prevent this type of tramp material, but not all clays are processed by this method.
Occasionally, clays that are air-floated are still flawed due to ineffective processing methods at the mill site. Milling machines are set to specific tolerances and are periodically inspected for consistency. As with any monitoring situation, human error can occur and allow larger nodules of limestone to enter the clay. In addition, if other materials, such as limestone, are crushed or milled in the same machines as clay, contamination can occur.
Potters should note any white specks in their dry or moist clay, as this can be an indication of limestone particles. However, in many instances, the clay has not been screened or wet mixed sufficiently, revealing other nodules of clay, flint, talc or feldspar. If only a few hard white nodules are found in the clay, it might be worth the time to remove them to prevent a potential lime pop. Some ceramics suppliers will screen clays to catch tramp material before it enters the mixing process. There is an additional charge for this procedure, but it more than pays for itself by decreasing defects. While lime pop is fairly rare, it is an imperfection that will easily ruin any pottery.
I would like to thank Eric Nedreberg of Resco Products Inc. for information used in this article.
From The Potter’s Studio Clay & Glaze Handbook by Jeff Zamek, Ceramics Consulting Services, 6 Glendale Woods Dr., Southampton, MA 01073, www.fixpots.com.