Spit outs are general terms for clay body and glaze defects that can occur in fired pottery. Specifically, concave or convex voids of approximately 1 m.m. to 3 m.m. diameter on the pottery surface can be caused by manganese dioxide found most likely in coarse mesh fireclays. 

The problem is noticed when an excess of reduction kiln atmosphere introduces more fuel than air in the combustion process. The resulting carbon monoxide atmosphere draws off an oxygen molecule from metallic coloring oxides found in clays. Large nodules of manganese dioxide under excessive reduction can result in its decomposition which can then act as an strong flux in the clay body. Nodules of manganese in the flux state during the firing can boil leaving ragged defects in the immediate area of the manganese deposit. Additionally, large particles of iron oxide also found in fireclays can result in smooth blotches of red/brown clay body surface defects. 

Finer mesh fireclays and less severe reduction kiln atmospheres go a long way to prevent these two types of clay body/glaze defects. Often fireclays are the least consistent part of any high temperature stoneware clay body formula however, they are needed to contribute “tooth” or stand up ability to the moist clay in the forming operation and increase the refractory quality and strength of the fired clay.

Overview

Any failure in a clay body or glaze represents hours of lost time and effort. A defective ceramic piece is not only lost but if a replacement is required a duplicate forming, glazing, and firing operation will be required along with the extra expenses of fuel and material. 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 corrections will insure a fast correction.

Many potters have experienced a semi elliptical 1/8” to ½”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 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 Limestone

Limestone contamination in moist clay comes most frequently from plaster wedging tables or plaster bats as 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 ½ 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 founded 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 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 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 allowing larger nodules of limestone to enter the clay. Additionally, 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 pops are fairly rare it is an imperfection that will easily ruin any pottery.

Acknowledgements:

I would like to thank Eric Nedreberg of Resco Products Inc. for information used in the article.

 Footnotes:

1. Ian J. McColm, Dictionary of Ceramic Science and Engineering, Second Edition, (Plenum Press, 1994)  

    page 188.

2. Frank and Janet Hamer, The Potter’s Dictionary of Materials and Techniques, (A& C Black, University

    Press 1986) page 43.Blog From, The Potter’s Studio Clay & Glaze Handbook by Jeff Zamek, Ceramics  

     Consulting Services, 6 Glendale Woods Dr., Southampton, MA 01073 www.fixpots.com

This common glaze defect appears as sharp edged crater in the fired glaze surface. It is often called “boiling” which is an appropriately vivid term. The potter will have to be a good detective to determine the actual cause as there are many possible reasons for this disruption of the glaze. Listed are several factors that can cause glaze blistering.

• Fast firing of the glaze kiln – many glazes go through a period of blistering before smoothing out. If the kiln is fired too fast the blisters are “frozen” in the glaze surface.
• Over firing or excessively long glaze firing – can result in lower constituent oxides boiling off the glaze. Ceramic materials melt under several different conditions which are, the higher the temperature, and the longer the firing time to temperature, the greater the melt.  
• Fast or incomplete bisque firing – can result in organic material being trapped in the clay. In the glaze firing it is released as a gas going through the molten glaze causing the blister.
• Direct flame or heat source impingement – on the glaze surface can cause a localized area of over firing causing a blister.
• Soluble salts in the clay body – can migrate to the surface as the clay dries, leaving a disruptive layer of sulfates releasing gas into the covering glaze.
• Higher levels of organic material in the clay body – periodically a clay will have higher than normal levels of organic material which the standard bisque firing cycle will not remove resulting in gas release at higher temperatures through the glaze surface.
• Firing the glaze below its maturing range – can result in a blistering glaze surface as it has not achieve a vitreous complete glaze surface.

When glaze blistering occurs the potter should researching all the possible factors and start by considering the most likely to investigate.

One of the most common defects occurs when the glaze contracts more than the underlying clay body as it cools in the kiln. In some instances the fired glaze can look intact but develops a fine series of lines in the glaze surface upon cooling. This reaction is called delayed crazing. It is important to note at temperature in the kiln almost every glaze fits correctly however, upon cooling the different rates of contraction in the glaze and clay body cause crazing.

Crazing can be corrected in some instances (if the craze lines are further apart than 1/2 inch) by adding additional amounts of silica to the formula which causes the glaze to contract less. However, if the craze lines are tightly packed a reformulation of the entire glaze is necessary. Changing the clay body in the expectation of arriving at a better glaze fit is often not practical if other glazes are fitting the existing clay body.