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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
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
- 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
- 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
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
- 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
- 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
- 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
- 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.
Out Bound Links
In Bound Links
Glaze Pinholing, Pitting
Analyze the causes of ceramic glaze pinholing and ...
A condition where fired glaze separates into clump...
In ceramics, a suspension is a mix of insoluble mi...
Glaze crawling on the inside and outside of a thin-walled cast piece.
G2415J Alberta Slip glaze on porcelain at cone 6. Center thinnest. Left: thicker. Right thicker yet and crawling (it has too much plastic clay in the recipe). Thin application looks best and does not crack on drying (so if cracking occurs during drying we know it is too thick).
By Tony Hansen