If a fired glaze looks like beads of water on a glass tabletop, you’re looking at crawling. The same forces of surface tension acting on beading water can also affect fluid molten glaze. Glaze crawling can have many possible causes, but in most cases you can try a few solutions simultaneously to eliminate the problem. First, make sure the problem is actually crawling and not another defect. The glaze should have rounded edges where it has drawn away from the exposed clay body. The bared patch can be quite dry or slightly shiny. In extreme cases, beads of glaze may have dropped off the pot onto the kiln shelf or other pots. Crawling occurs when the unfired glaze and the underlying clay body do not build and hold a uniform continuous bond. If a glaze has the potential to crawl, thicker applications will exacerbate the problem. Often the simplest solution is a thinner application. If this does not work, other steps can be taken to identify and eliminate the problem.
Grease and Dust
Greasy or dusty surfaces disrupt the clay body and glaze bond. To avoid dust problems, blow any dry particles off green-ware before glazing, and place a cover over the bisqueware immediately after unloading from a kiln. To ensure a grease-free surface, always wash your hands before touching green-or bisqueware.
Low Density and High Shrinkage
Low-density and high-shrinkage-rate glaze materials can also cause crawling. For example, magnesium carbonate, a light fluffy material, has a low density. Bentonite and other types of clays shrink excessively. Both can contribute to crawling. Low-density glaze materials do not compact well on the clay body when the water leaves the glaze, and a soft powdery surface forms. Once the water evaporates, the low-density materials remain “unpacked” or loose, causing an insufficient bond with the clay body underneath the glaze. Often glazes containing low-density materials are difficult to handle and load into a kiln without damaging the surface. Materials that have excessive shrinkage rates, such as Gerstley borate, colemanite, soda ash and borax, can hold massive amounts of mechanical or chemical water. Zinc in a glaze can also cause crawling due to its shrinkage rate at high temperatures. A glaze material that is overmilled can also cause crawling due to the increased amount of surface area that must be made wet for glaze application. To correct high-shrinkage-glaze problems, use calcined clay for half the clay component in the recipe. Another alternative is to use a clay that shrinks less than the original clay component of the glaze. For example, if the original recipe calls for ball clay (a high-shrinkage clay), kaolin might be substituted. Less glaze shrinkage at this stage can eliminate crawling as the firing glaze begins to mature. Often frits can be used (less shrinkage due to the calcining process) in place of Gersdey borate or colemanite. Shrinkage rates can also be reduced by using the same material in a coarser grind. In some situations, lesser amounts of the high-shrinkage material can be used in the glaze. Another solution might be to add Veegum CER or CMC to the glaze. Both materials act as binders, keeping the high-shrinkage and low-density materials together long enough for the sintering or melting process to take hold.
Crawling can occur when the bisque temperature is not high enough for correct glaze absorption. The ware is too absorbent, causing a thick buildup of glaze. Thick glaze applications are more likely to crack upon drying, which can cause crawling. At the opposite extreme, too high a bisque temperature or uneven hot spots on the bisqueware can cause the glaze to adhere incompletely, which then results in crawling at the firing stage. Bisquing problems can be overcome by choosing a temperature for the clay body that will allow the glaze to build up uniformly and to the proper thickness, and by firing the bisque kiln evenly throughout. Most stoneware clay bodies can be bisqued at Cone 06 (1 830°F). Porcelain bodies usually require a Cone 04 (l940°F) bisque due to their high-refractory-clay content.
Loading ware with wet glazes in a fast-firing kiln can cause crawling. Some potters will rush to start a firing even though the water has not fully evaporated from the glazed ware. Fast heating can cause the water in the glaze to turn into steam; as it expands, it blows off parts of the glaze. To avoid this problem, place glazed ware in the kiln when it is dry to the touch, then increase the kiln temperature slowly (from 212°F to 1l00°F) to drive off mechanical and chemical water in the glaze at a safe rate. Applying once-fired glazes by spraying decreases the amount of water needed and reduces the chances of the raw glaze shrinking and eventually crawling off the pot. Again, slow heating in the 200°F to 11 OO°F ranges will allow any water present in the clay body and glaze to be released safely.
Multiple applications of glaze can cause crawling, particularly if the base glaze or the overlapping glaze dries to a powdery condition. When this happens, the mechanical bond between the overlapping glazes is compromised. The powdery glaze acts like “ball bearings,” resulting in a nongripping condition. If the materials that are causing the soft or powdery surfaces cannot be replaced, additions of Veegum CER or CMC should be used in the base glaze or the overlapping glaze, or in both.
Refractory or Dusty Underglaze Washes
Frequently, underglaze stains or metallic coloring oxides are applied to green-or bisqueware, then covered with a clear or semitransparent glaze; however, some stains and oxides are more refractory than others, and do not readily go into a melt, particularly those containing manganese, chrome, cobalt and nickel. Refractory underglazes will cause the covering glaze area to crawl during the firing. occur due to the glaze not bonding prop-erly with the underlying clay body. The solution is simple: When glazing thin-walled pots, wait for the first glaze application to dry completely before applying the second glaze. Sprayed glazes are also likely to crawl if the spraying continues when the first layers are still wet. As excess water in the glaze evaporates, it leaves dry glaze particles loosely compacted. To avoid this problem, spray glaze until the surface becomes moist, then stop. The glaze should pack or compress itself on the clay body surface, resulting in a stable bond. If the glaze is sprayed from too far a distance, a dry, dusty, lightly packed glaze layer can develop as well. Check the sprayed Ferro frit 3195 can be used to flux or melt refractory washes; try a mix of 60% stain or metallic oxide to 40% Ferro frit 3195 for a Cone 06 (1 830°F) firing. For Cone 6 (2232°F), use 70% stain or metallic oxide to 30% Ferro frit 3195. For a Cone 9 (2336°F) firing, mix 80% stain or metallic oxide to 20% Ferro frit 3195. Once the fritted underglaze has dried on the raw or bisqued clay, it should develop a smooth, A thick coat from overlapping glaze can cause crawling. Hard surface that will ensure a stable bond with the covering glaze. If the underglaze wash is dusty; crawling of the overlying glaze is likely to take place. To tack down dusty underglazes, add 0.5% to 2% Veegum CER or CMC binders based on the dry weight of the underglaze wash; for example, to 100 grams of blue stain, add 2 grams of CMC. Water is added until the mixture is a “watercolor” consistency, then the wet mix is screened through an 80-mesh sieve. The binder also enables the underglaze to flow off the brush in a smooth motion, preventing the brushstroke from skipping across the clay or bisque surface. In situations where an underglaze wash is both refractory and dusty, frit and binders can both be added.
Excessive Water Penetration
When thin-walled pots are glazed first only on one side, the water in the glaze will penetrate through the pot wall, causing a damp or wet clay surface on the opposite unglazed side of the pot. If glaze is then applied to the wet surface, crawling can glaze when it is dry by touching or lightly rubbing; it should not dust off excessively on the fingers. The raw material makeup of a glaze recipe determines the amount of water required for successful spraying. In short, some glazes need more water for good spraying results, some less. To obtain the best results, practice spraying with differ-ent concentrations of water to glaze and at varying distances from the clay surface.
Often found in high-iron earthenware clays and in fireclays, soluble materials can also cause crawling. As the clay dries, these soluble salts will leach to the surface, forming a fuzzy white layer, which does not disappear in the bisque firing. The glaze layer will then make insufficient contact with the clay body, causing the glaze to mature in “mid air.” Washing or scraping the salt deposits off the bone-dry or bisqued clay is a waste of time. Instead, add barium carbonate to the clay body (0.25% to 2% based on the weight of the dry ingredients) to prevent the formation of soluble salts.
A Type 1, Type 3 or Type 4 (0.06% to 0.25% based on the weight of the dry ingredients) will also neutralize soluble salts. Any changes in solubility, pH levels or organic conditions can alter the glaze’s bonding capacity to green-or bisqueware. Excessive ball milling or grinding of the glaze can release soluble materials into the batch. An increase or decrease in the water pH levels can develop due to a change at the water source or to the breakdown of raw materials. Glazes stored in the liquid state for long periods can develop bacteria or mold growth. To avoid problems introduced by such changes, use insoluble glaze materials when possible, or add acid-based or alkaline material to adjust the glaze water pH level. Adding one or two drops of bleach per gallon of glaze can coun-teract mold or bacterial growth in the glaze.
Tension Glazes that contain large amounts of alumina, tin or any of the zirconium silicate opacifiers can have high surface tension, causing the fired glaze to become “stiff” when molten. Often, glazes containing such materials are opaque when fired; some have matt surface textures. High-surface-tension glazes will con-dense or pull into themselves, exposing areas of bare clay. Conversely, a glaze with low surface tension flows out, filling cracks and voids in the underlying clay body. To correct, reduce the percentages of alumina, tin or other high-surface-tension-producing materials. For the most part, however, glaze crawling is not due to high surface tension, but to other factors dis-cussed previously-glaze adhesion, thickness and raw-material-shrinkage rates. While the causes of glaze crawling can be attributed to several factors, the fired results are the same. Identifying the specific cause will make the solution much easier to find. When faced with crawling, ask these questions: Was the bisque surface dusty or greasy before glazing? Was the glaze loose, dusty or cracked after it had dried on the pot? Does the glaze include high-surface-tension-producing raw materials? All are conditions that can suggest a potential crawling problem. Knowing where the problem started will bring about a faster fix.