by Jeff Zamek
Many potters have experienced a semi-elliptical J- to 1⁄2-inch crack in their bisque or fired ware. Upon peeling back the defect, a conical hole reveals either a black or white nodule at the bottom. This is called a lime pop. Lime pops occur when moisture in the air comes into contact with a carbon- ized lime nodule, causing it to expand in an unyielding fired clay body. Unfortunately, this defect can occur shortly after removing glazed or unglazed work from the kiln, or years later as lime expands in the form of calcium hydroxide. In some types of building brick, lime pops can 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 pow- der, the forces of expansion are not sufficient to crack the clay. In low-fire white clay bodies, powdered limestone (usually in the form of whiting or dolomite) is often added to prevent glaze crazing.1 When used in earthenware glazes, large percentages of limestone can cause crystal growth. In high-temperature glazes, powdered limestone acts as a flux bringing other glaze materials into a melt.2 Lime in all of these forms is beneficial to the working and firing properties of clay and glazes. The problem arises when the lime is present in small chunks.
1. Dark area of fired lime at the bottom of a ta- pering hole. Moisture expansion of lime caused the glaze surface to crack.
3. Cross section of fired clay body showing voids containing white particles of limestone.
Cover a plaster wedging table with canvas to prevent plaster chips from entering clay during reprocessing or wedging operations.
Wedging Tables and Bats
Limestone contamination in moist clay comesmost frequently from the plaster found in wedging tables and throwing bats. Plaster is comprised of lime, gypsum and water, and 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, staple a canvas cloth on top of the plaster wedg- ing board. Bats should also be carefully inspected for any soft spots or concave areas, which could indicate past introduction of plaster chips into the moist clay.
Raw clay can also be a potential source of lime particles. Some clays form next to strata of lime- stone 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 accom- pany the clay to the milling operation. Contamina- tion can also be introduced into clay as 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 pre- vent this type of tramp material, but not all clays are processed by this method. Occasionally, air-floated clays are still flawed due to ineffective processing methods at the mining site. Additionally, if other ma- terials such as limestone are crushed or milled in the same machines as clay, contamination can occur.
Plaster bats should be kept in good condition as a damaged section of a plaster bat can deposit chips into moist clay.
Pay attention to any white specks in dry or moist clay, as these may be limestone particles. In many instances, however, the clay has not been screened or wet mixed sufficiently, and these white specks may just be nodules of clay, flint, talc or feldspar. If only a few hard white nodules are found, remove them to prevent a potential lime pop. Some ceram- ics suppliers screen clays to catch tramp material before it enters the mixing process. There is an ad- ditional charge for this procedure, but it more than pays for itself by decreasing defects. While lime pops are fairly rare, they are imperfections that eas- ily ruin any pottery.
Jeff Zamek is a ceramics consultant and frequent contributor. For more information, visit his website at www.fixpots.com. Special thanks to Eric Nedreberg of Resco Products Inc. for information used in the article.
1 Ian J. McColm, Dictionary of Ceramic Science and Engineering, Second Edition, Plenum Press, 1994.
2 Frank and Janet Hamer, The Potter’s Dictionary of Materials and Techniques, A&C Black, University Press 1986.