What makes a glaze crawl

If a glaze can be applied more thinly, you should do so. It is practical to 'gel' the glaze slightly i. 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.

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. Preheating the bisque may cause escaping steam to rupture the bond with the ware. This base layer can be fired on in the bisque. It might be enough to prevent crawling when the piece is glazed later.

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 C 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.

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.

Make sure ware is absolutely dry before firing. Mix under-glaze stains with a flux medium so that over lying glazes can 'wet' them and form a glassy bond. This cone 6 white glaze is crawling on the inside and outside of a thin-walled cast piece.

This happened because the thick glaze application took a long time to dry, this extended period, coupled with the ability of the thicker glaze layer to assert its shrinkage, compromised the fragile bond between dried glaze and fairly smooth body. There are several measures that can be taken to solve this problem. The ware could be heated before glazing, the glaze applied thinner, or glazing the inside and outside could be done as separate operations with a drying period between.

This is GJ Alberta Slip glaze on porcelain at cone 6. Why did the one on the right crawl? Left: thinnest application. Middle: thicker.

Right thicker yet and crawling. All of these use a calcine :raw mix of Alberta Slip in the recipe. While that appears fine for the two on the left, more calcine is needed to reduce shrinkage for the glaze on the right perhaps calcine:raw.

This is a good demonstration of the need to adjust raw clay content for any glaze that tends to crack on drying. This is an example of how a glaze that contains too much plastic clay has been applied too thick.

It shrinks and cracks during drying and is guaranteed to crawl. This is raw Alberta Slip. To solve this problem you need to tune a mix of raw and calcine material. Enough raw is needed to suspend the slurry and dry it to a hard surface, but enough calcine is needed to keep the shrinkage low enough that this cracking does not happen. The Alberta Slip website has a page about how to do the calcining. It was spray applied on the dried bowl no bisque fire an was too thick not to mention under fired.

But the main problem was a glaze recipe having too high a clay content. For example, you can buy calcined kaolin to mix with raw kaolin. Or you can calcine the clay in bowls in your kiln by firing it to about F. The glaze on the right is crawling at the inside corner. Multiple factors contribute. The angle between the wall and base is sharper. A thicker layer of glaze has collected there the thicker it is the more power it has to impose a crack as it shrinks during drying.

It also shrinks more during drying because it has a higher water content. But the leading cause: Its high raw clay content increases drying shrinkage. Calcining part of the raw clay destroys its affinity for water which is what makes it plastic , this is an effective way to deal with this. Or doing a little chemistry to source some of the Al 2 O 3 from materials other than clay e. It induces crawling. It also mattes the glaze because it sources MgO.

Example of glaze crawling on the inside of a stoneware mug. Notice how thick it is. Thickly applied glazes have more ability to assert their shrinkage during drying and thus compromise their bond with the body below. The cracks that appear become bare patches after firing. These mugs are quite thin walled. A glaze has just been applied to the inside. Notice how it has water logged the bisque you can see the contrast at the base, where the clay is a little thicker and has not changed color yet.

Although there may be enough absorbency that a glaze could be applied now, it would still not be a good idea because it would completely waterlog the piece and result in a very long drying time. This is bad, not only because of process logistics, but also because slow drying glazes almost always crack and lift from the bisque causing crawling.

Example of Alberta Slip which has been sprayed on dry ware and single fired. This happened because the slip shrunk during drying creating a network of cracks. These cracks become the crawl-points during firing. Are you really sure the problem is with the materials? But then it started crawling. I tried mixes with new materials and the old ones. Still crawled. The problem? What was I thinking? I actually do not know! But I am now calcining Ravenscrag as appropriate as documented at ravenscrag.

The glazed item is carefully loaded into the kiln for the glaze firing. It must not touch other pots or the glazes will melt together, fusing the pots permanently. The kiln is heated slowly to the proper temperature to bring the clay and glazes to maturity, then it is slowly cooled again. Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.

Crazed ceramic glazes have a network of cracks. And you can add hazards to you and customers of your ware by the way you use them. Use low fire, earthenware clay that can be bisque fired, and glaze fired in the same temperature range. For example, using clay that is happy being bisque and glaze fired at cone 05 should be fine. Remember that you will need to use a low fire glaze too. For a standard pottery piece, two coats of glaze are enough; one underglaze and an overglaze is enough to make your pottery look amazing.

You should consider the clay body of the piece you are about to glaze and the required temperature for the glazes. Note that excessive glazing can ruin the beauty of your ceramic. Fluid melt glazes, or those having high surface tension at melt stage, can blister on firing if applied too thick. Glazes having sufficient clay to produce excessive shrinkage on drying will crack and crawl during firing if applied too thick. Fluid melt glazes will run off ware if applied too thick. Pottery can be reglazed and refried multiple times.