Crawling is where the molten glaze withdraws into 'islands' leaving bare clay patches. The edges of the islands are thickened and smoothly rounded. In moderate cases there are only a few bare patches of clay, in severe cases the glaze forms beads on the clay surface and drips off onto the shelf. The problem is most prevalent in once-fire ware.
Is the glaze shrinking too much during drying?
If the dried glaze forms cracks (or in serious cases flakes that peel and curl up at the edges) it is a sign that the glaze is shrinking too much. These fault lines provide places for the crawling to start. There are a number of possible contributors:
• If very fine-particled materials are present (i.e. zinc, bone ash, light magnesium carbonate) these will contribute to higher shrinkage during drying. Try using calcined zinc, synthetic bone ash or another source of calcia, talc or dolomite to source magnesia instead of magnesium carbonate.
• It is normal to see 20% clays (ball clay, kaolin). If significantly more is present try using a less plastic clay (i.e. kaolin instead of ball clay, low plasticity kaolin instead of high plasticity kaolin, or a mix of calcined and raw kaolin). If you are using Gerstley borate, try a boron frit. You may need to do calculations to make these adjustments. Ultimately you need to tune the glaze's clay content to achieve a compromise of good hardness and minimal shrinkage.
• If a glaze has been ball milled for too long it may shrink excessively (for example, zircon opacified glazes can be ground more finely than tin ones). Highly ground glazes may produce a fluffy lay down.
• If a slurry has flocculated (due to changes in water, dry material additions like iron oxide, or addition of an acid, epsom salts, calcium chloride, etc) it will require more water to achieve the same flow and will therefore shrink more during drying and require a longer period to dry. Try using distilled water. Always measure the specific gravity to maintain solids content and use deflocculants/flocculants if necessary to thin/thicken the slurry (you can remove water from an existing glaze slurry by pouring some on a plaster batt, then mixing the water-reduced mass back in).
• Gerstley Borate is plastic and therefore contributes to glaze shrinkage, especially if the recipe already contains kaolin or ball clay. It also tends to gel glazes so they need excessive water. Use boron stains instead.
It is possible to create glaze slurries that gel and flow extremely well using the right kaolin (i.e. EPK) in adequate amounts. This requires a glaze base whose other materials do not contribute too much Al2O3. We have a separate article on glaze slurry properties that deals with this (see links).
Is the glaze's dry-bond with the ware surface inadequate?
• The mechanism of the bond is simply one of physical contact, the roughness of the ware surface combined with the hardness of the glaze determines its ability to 'hang on'. Some surfaces can be very smooth (e.g. slip cast surfaces). To give the glaze better ability to hang on, there should be some clay in the glaze mix to both suspend the slurry and toughen the dried layer. If ware is also excessively powdery to handle this is a signal to incorporate more plastic clay, add a little bentonite, or add a hardener like gum.
• Add gum to glaze to bond better to bisque.
• If a glaze is deflocculated it may lack the necessary fluidity to run into tiny surface irregularities in the bisque and establish a firm foothold.
• Wetting agents are available and can be added to the slurry to improve bond.
Does application technique or handling compromise the fragile glaze-body bond?
• Make sure ware is clean and dust free, even oil from ones skin can affect glaze bond.
• If glaze is applied too thickly the forces imposed by its shrinkage will overcome its ability to maintain a bond with the ware surface (especially inside corners or at sudden discontinuities). If a glaze can be applied more thinly, you should do so.
• Use a fountain glazing machine to do the insides of bowls and containers to achieve a thinner layer.
• If glaze needs to be applied in a thick layer, you can achieve a lower water content by deflocculating the glaze (i.e. with some sodium silicate or Darvan), however it may then tend to dry very slowly or form drips that crack and peel and instigate crawling.
• When applying the glaze in the normal layer thickness be careful to prevent drips that form thicker sections that can crack away during drying. It is practical to 'gel' the glaze slightly (i.e. with vinegar, Epsom salts) so that it 'stays put' after dipping or pouring.
• If a double-layer of glaze needs to be applied be careful that the second does not shrink excessively and pull at the first, compromising its bond with the body. If possible, the upper layer should have less clay and lower shrinkage and should dry quickly. It may be necessary to bisque each layer on before applying the next. Double-layering typical raw art and pottery glazes is difficult, special consideration must be given. If you have successfully done it in the past without any special attention then you may have simply been very lucky.
• When doing double-layer glazing be careful that the second layer is not flocculated (with an associated high water content). This will rewet the first layer and loosen it from the body. Adding iron oxide, for example, to a glaze will often flocculate it and require the addition of much more water to restore the same fluidity.
• Spraying glaze on in such a way that the glaze-body bond is repeatedly dried and rewetted could produce shrinkage-expansion cycle that compromises a glaze-bisque bond that could otherwise withstand one drying-shrink cycle.
• Force-drying of the ware can make the glaze visibly crack when it otherwise would not (slower shrinkage associated with slower drying gives it the glaze time to ease body interface tension by micro cracking). Preheating the bisque may cause escaping steam to rupture the bond with the ware.
• Rough handling of ware can compromise sections of the glaze body bond.
• Consider pouring a thin glaze slurry into the mold of a just-drained piece (perhaps a minute or two after the mold has been drained) and immediately pouring it out again. This base layer can be fired on in the bisque. It might be enough to prevent crawling when the piece is glazed later.
Is the glaze drying too slow?
• If the glaze dries too slowly the most fragile stages of adhesion are extended and cracks in the dried glaze layer can appear. Bubbles in the wet glaze layer can also form during the drying, these become areas of no bond with the underlying body and therefore can instigate crawling during melting. This can occur if ware is very thin, glaze has a high water content, or if ware is already wet when glaze is applied. To speed up drying try preheating the bisque (in a kiln to 150C or more if necessary), doing separate interior and exterior glazing, make ware thicker and better able to absorb water or apply the glaze in a thinner layer.
Is the ware once-fire?
• Once-fired ware is much more prone to crawling because the mechanical glaze-body bond is more difficult to achieve and maintain. If glaze is applied to leather hard ware it must shrink with the body. During the early stages of firing the ware also goes through volume changes and chemical changes that generate gases, these make it difficult for the glaze to hang on.
• When glaze is applied to leather hard ware you must be able to tune its shrinkage by adjusting the amounts and nature of the clays in the recipe (calculations may be needed).
• Once-fire ware must not be fired too quickly, especially through the water-smoking period. Make sure ware is absolutely dry before firing.
• In damp conditions the powdery layer may reabsorb water from the air causing slight expansion and breaking of the adhesion.
Is the problem happening during firing?
• If glaze is applied over stains or oxides that lack flux (e.g. chrome pinks, manganese types, greens, cobalt aluminate) they will act to prevent bonding with the underlying body. Mix under-glaze stains with a flux medium so that over lying glazes can 'wet' them and form a glassy bond.
• If the glazed ware is put into the kiln wet and therefore dried quickly during the early stages of firing the glaze layer will tend to crack and curl and crawling can occur.
• If glazed ware is put into a kiln containing heavy damp ware such that early stages of firing occur in very high humidity conditions the glaze could be rewetted and forced through an expansion-shrinkage cycle that could affect its bond with the body.
• If a glaze contains significant organic materials (i.e. gums, binders) that gas off excessively during firing the glaze-body bond may be affected. Decomposition of materials like whiting can also generate significant amounts of gas within the glaze layer (try switching to wollastonite, it supplies SiO2 also and will allow you to reduce the silica content accordingly).
• Raw zinc oxide is very fine and tends to pull a glaze together during firing, use calcined zinc instead.
• If the glaze contains significant zircon opacifier, alumina, some stains, magnesium carbonate, the melt may be much 'stiffer' and flow less. This can affect its ability to resist crawling.
• Watch out for glazes with slightly soluble materials like Gerstley Borate or wood ash. With the former the partly soluble and the soluble portion tends to be the borate which will be absorbed into the bisque during application and then during firing creates a highly fluid layer between the body and the less developed glaze and thereby prevents adhesion of the glaze to the body (use frit to source boron instead). In addition soluble materials tend to flocculate (thicken) the slurry and attempts to thin them result in higher water content and therefore increased shrinkage.
• If the bisque firing is reduced or not adequately oxidized and excessive gases are generated during certain stages of the glaze firing, these can affect the glaze-body bond.
• If bisque ware is dense and non-absorbent (fired too high) it may not form a good bond with the glaze.
• The chemistry of glaze may be such that the surface tension of the melt encourages crawling (e.g. high alumina, high tin, significant chrome/manganese colorants, lack of fluxes of low surface tension).
Is there a problem with the body?
• If the clay body contains soluble salts that come to the surface during drying, these can affect the fired melt's ability to form a glassy bond with the body. Precipitate these salts with a small addition of barium carbonate to the body (for information on how this works search for Barium Carbonate in the materials section).
• An noted above, if the body surface is too smooth, the glaze may not be able to adhere properly.