Crazing pottery

I will go over what is Crazing in Ceramics and how to prevent it.

If you are a collector of vintage then you've probably heard of crazing but do you know what it really is? In this post I'll cover the following common questions:. What is crazing? What causes crazing? Are dishes with crazing safe to use? Can I remove crazing? Does crazing affect the value?

Crazing pottery

This article discusses polymer crazing , which occurs in regions of high hydrostatic tension, or in regions of very localized yielding , which leads to the formation of interpenetrating microvoids and small fibrils. If an applied tensile load is sufficient, these bridges elongate and break, causing the microvoids to grow and coalesce; as microvoids coalesce, cracks begin to form. Crazing occurs in polymers, because the material is held together by a combination of weaker Van der Waals forces and stronger covalent bonds. Sufficient local stress overcomes the Van der Waals force, allowing a narrow gap. Once the slack is taken out of the backbone chain, covalent bonds holding the chain together hinder further widening of the gap. The gaps in a craze are microscopic in size. Crazes can be seen because light reflects off the surfaces of the gaps. The gaps are bridged by fine filament called fibrils, which are molecules of the stretched backbone chain. The fibrils are only a few nanometers in diameter, and cannot be seen with a light microscope, but are visible with an electron microscope. The thickness profile of a crazing is like a sewing needle: the very tip of the crazing may be as thin as several atoms.

Band-aid approaches just won't work, the thermal expansion of the clay and glaze must match closely. The crazing pottery colour is caused by light-scattering from the crazes.

At the Hewitt Pottery we have been developing some glazes using local granites with a high felspatic content. The glazes are beautiful and sparkly but we have experienced some issues with crazing, so during our recent snowstorm, I spent some time reading through books and looking online to see what I could glean. I wanted to share the sum of what I have learned here in three sections: 1 Why is crazing a concern? Crazing can be an attractive feature of a pot and is often called "crackle" when intentionally used, such as on this tea bowl: Intentional crazing, or "white crackle" glaze, on a tea bowl by Richard Brandt. There are reasons why crazing is not ideal for functional pottery, however. Crazed pots may leak if the clay body is not totally vitrified, and potentially be unsanitary as bacteria can grow in the cracks. Structurally, crazing is also an issue as Michael Cardew points out in Pioneer Pottery , p.

Many ceramic artists deliberately create faults in their glaze surfaces to achieve a particular aesthetic. But, of course, there are some cases in which a glaze must be perfect for reasons of safety or hygiene. Read on to get some expert pointers on how to solve five of the most common pottery glaze defects such as crawling, shown at above. Crazing is the most common glaze defect, and normally the easiest to correct. In both crazing and shivering the eradication of problems relies on matching the thermal expansion characteristics of both body and glaze. In practice, the most effective ways to correct crazing are:. Shivering is the most problematic of glaze defects, particularly in any functional ware, since slivers of glaze from the edges can drop into food or drink. These slivers are usually small, sharp, jagged-edged pieces that detach themselves from the body.

Crazing pottery

In ceramics, one person's fault is another person's fancy. While crazing is technically a glaze fault , it can also be a very beautiful effect, and many potters seek to increase rather than reduce crazing. Fortunately, this post can apply to both fancies because learning how to reduce crazing will also give you the knowledge on how to increase crazing! In this post, an excerpt from her book Special Effect Glazes , Linda Bloomfield tells you all you need to know to increase or reduce crazing in a glaze! We will learn about crackle by first considering the options available for eliminating crazing. While gaining an understanding of how to either reduce or increase crazing, at the same time we will learn how to make crackle glazes. There are several ways to correct crazing. However, changing only one material may change the appearance of the glaze, making it more glossy or matte.

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The straight lines on the chart represent alumina:silica ratios of matt , semi-matt and shiny, crazed glaze. August Is Crazing a Sign of Weakness in Dishes? Colbeck, John. Please help improve this article by adding citations to reliable sources. One of my teapots from Firing 96 at the Hewitt Pottery. The thicker the glaze, the more liable it is to craze. February CaO 1. Intentional crazing, or "white crackle" glaze, on a tea bowl by Richard Brandt.

As an affiliate, we may earn a commission from qualifying purchases. We get commissions for purchases made through links on this website from Amazon and other third parties. Crazing in pottery glaze is a network of very fine cracks that cover the glaze on a piece of ceramics.

Can Crazing Lead to Leaching of Chemicals? Heating and then cooling too rapidly can cause the glaze to shrink too quickly and cracks appear more readily. This has the effect of moving along the dashed blue line in the graph and crossing the red line from the crazed to the glossy, uncrazed area. This can be a tricky process and may take some experimentation to get the right balance. If you have tried all of the above and are still getting Crazing from your Commercial Glazes or Clay, you will need to change the brand of your Glaze or Clay. It is true that crazing will be evident at low-fire with a much smaller difference between clay and glaze expansions than with stoneware. Third ed. Can crazing coax harmful substances out of your dishware and into your food? These little cracks can become a favorite hangout spot for bacteria, triggering possible health concerns. On the Stull map, these Snowflake crackle glazes lie on the semi-matte line, while glossy crackle glazes low in silica and alumina lie in the crazed area in the bottom left-hand corner. If an applied tensile load is sufficient, these bridges elongate and break, causing the microvoids to grow and coalesce; as microvoids coalesce, cracks begin to form. In addition, when crazing occurs, one will typically not observe "necking," or concentration of force upon one spot in a material.