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Glaze Crawling

The molten glaze withdraws into 'islands' leaving bare clay patches. The edges of the islands are thickened and smoothly rounded. In severe cases the glaze forms beads on the clay surface and drips off onto the shelf. More prevalent in once-fire ware.

Is the problem already evident during application or drying?

Is the glaze shrinking too much?

If the dried glaze forms flakes that peel and curl up at the edges it is a sign that the glaze is shrinking too much. 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). 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 (Magic of Fire has a chapter on slip glazes).
• If a glaze has been ball milled for too long it may shrink excessively.
• If a slurry has flocculated (due to changes in water or dry materials or addition of an acid, epson 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.

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'. There should be some clay in the 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.

Does application technique compromise the fragile glaze-body bond?

• Make sure ware is clean and dust free.
• If glaze is applied too thick the forces imposed by its shrinkage will overcome its ability to maintain a bond with the ware surface. If glaze needs to be applied thick, 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.
• Apply the glaze in a thinner layer if possible, but 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. If necessary, bisque each layer on before applying the next.
• 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 crack when it otherwise would not. Preheating the bisque may cause escaping steam to rupture the bond with the ware.

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. 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, doing separate interior and exterior glazing, make ware thicker and better able to absorb water, apply the glaze in a thinner layer.

Is the ware once-fire?

• Once-fired ware is much more prone to crawling because the 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 leatherhard 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.

Is the problem happening during firing?

• If glaze is applied over stains or oxides that lack flux (e.g. chrome, manganese types) 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 dried too quickly the glaze layer will tend to crack and curl and crawling with 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 flint 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, 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 Gerstley Borate. It is partly soluble and the soluble portion tends to be borate. It will be absorbed into the bisque during application and then during firing it 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.
• 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, contains significant chrome/manganese colorants).

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. The Magic of Fire II contains information on how to test a body for soluble salts.

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