WHAT CAUSES GLAZE TO CRACKLE ON POTTERY?
I am sure you have seen a crackle glaze on a ceramic before. You might have even bought one. As a beginner in pottery, you might ask yourself how come a glazing mistake turns into a masterpiece. Your answers are going to be answered in this article. Crackle glaze is caused by crazing. It is a type of pottery glaze that is intentionally crazed. Glazes that have crazed show a fine pattern of cracks in the surface of the glaze. It is very easy to detect by simply breathing on the piece and fogging the glaze surface. Crazing is a result of a discrepancy between the coefficient of expansion of the glaze and the body of the clay. When the glaze is expanding more than the clay body, there will be crazing.
WHAT ARE CRACKLE GLAZES?
Crackle Glazes are often a network of lines or cracks in the fired glazed surface. As earlier said, it occurs when a glaze is under tension. A craze pattern can develop immediately after removal from the kiln or years later. Some pottery pieces’ craze/crackle more or less than other pottery pieces depending on the clay body or glaze ratio/ lot, pottery usage and exposure to extreme temperature, etc. As said earlier, crazing is deemed to be a glazing defect, but it can be corrected. It can be corrected by reducing the difference in the expansion of the glaze or switching to a higher expansion clay body.
Before we talk about the recipe for a great glaze crackle, let’s take some time to discuss the constituents of a glaze. You will need to know the elements of a glaze to understand the way to make an amazing crackle glaze for your pottery.
CONSTITUENTS OF CERAMIC GLAZES
|CONSTITUENTS||RELEVANCE IN A GLAZE|
|Glass Former||It is used as the sand component of the glaze|
|Flux||This is the melting agent that binds the crackle glaze to the body of the clay|
|Colorant||It brings out the true beauty in every glaze|
|Refractory Component||It helps to stiffen the glaze. It keeps the pottery from breaking when being fired.|
The glass former component is also known as Silica and it is mostly sand as this quantifiable naturally forms glass when heated to extremely high temperatures. Silica sand is the desired material to use for the sand component. It is easily available through commercial mining and it saves cost. Silica can be gotten naturally from quartz, sandstone, sand, or flint, or it can be manufactured as silica oxide. When making your crackle glaze, you can add products like quartz or flint to increase the glaze adhesion ration to the clay body of your piece. Because Silica is a mined product, it is deemed to be purer than the sand that comes from surface sources. This ensures that the glaze manufacturer the ability to control all aspects of pottery when preparing different types of crackle glazes.
Flux is the melting agent that binds the crackle glaze to the body of the clay. It helps in lowering the melting point of silica. Flux is necessary for the glaze because it acts as a glue to hold the glaze onto the pottery piece. Without it, the glaze will melt out during the high temperatures involved in the kiln. Glaze must adhere to the piece during firing or be redone completely. If the glaze does not adhere properly, then the piece may be ruined. Sodium, calcium, and potassium are common materials added to glaze as flux.
The colorant is the beautifier element in a glaze. It brings out the true beauty in every glaze. Colorants can withstand high temperatures without burning off because they are mostly made from metallic oxides—a medium that can also affect the melting point of the glaze. Colorants can be added to a glaze to increase or decrease opacity and create pretty colors for your ceramic piece. Once the colorants melt on the body of your piece, it creates different hues that make decorating pottery with glazes so satisfying.
The final ingredient in any basic glaze mix is the refractory component. This refractory component is mostly aluminum which acts as the stiffening agent. This component is in charge of lowering the temperature at which the silica sand will melt into glass. It is an important part of the mix because it keeps the pottery piece from breaking under the kiln high temperatures. Without the presence of aluminum, the glaze will slide off any vertical piece. Lowering the temperature of the glass-forming ingredient does not affect the quality of the final finish. Not only does the refractory element of a glaze stiffen the glaze, but it also helps to dissolve air pockets that can form in the firing process.
Recipe for Crackle Glaze:
White Crackle Glaze with Cone 06 Oxidation
Glaze Material Percentage: Frit 3134 (Ferro) 85%, Kaolin 15
Kuan Crackle Glaze with Cone 10 Oxidation
Glaze Material Percentage: Custer Feldspar79.0 %, Whiting 9.5%, Silica 9.5% and then Bone Ash 2.0%. You can later add: Zircopax 10% and Bentonite 2.0%
Then fire to your pottery to cone 10 and you can decide to stain it with some Indian Ink.
If you are limited to resources, you can make a simple crackle glaze by using Gerstley Borate of 65%, Tennesse ball clay 5%, Nepheline Syenite 15%, Tin Oxide 10% and a flux agent (silica) of 5%.
If you are using a unique glaze and it can’t be changed or modified, try another clay body. You can try a simple Cone 06-04 white clay made from 50 parts ball clay, and probably a white clay. The white clay helps glazes aid crackle.
Use a glaze with a higher coefficient of expansion to the clay body of your piece to make the crackle or crazing effect easier. To achieve the perfect crackle desired, I will advise you to get a premixed crackle glaze from amazon by clicking here.
In this article, I have discussed the cause of crackle glaze on pottery pieces, what crackle glazes are, basic elements of a glaze and recipes for crackle glaze. With this article, I believe you are equipped with adequate resources to start making your crackle glaze.