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Demonstrating Glaze Fit Issues to Students
Section: Glazes, Subsection: Thermal Expansion
Glaze and body can both be adjusted to solve crazing and shivering problems. This describes a simple project to create body glaze combinations guaranteed to craze and shiver to demonstrate the principles involved.
At the risk of sounding like a broken record, I want to flog the issue of crazing again. I continually meet students who have no idea about what it is. This is incredible to me. No piece of functional ware should be crazed. Period. This is our duty to customers.
Crazing is a technical issue and the buck stops with you. You cannot put the whole burden of dealing with it on your suppliers, you really need to understand what it is and what causes it to be able to exercise the necessary diligence. I won't review it all here, but suffice it to say that crazing happens when you have a size 5 glaze stretched on a size 6 pot. The glaze then cracks to relieve the stress. The stress develops during cooling (with its associated shrinkage) in the kiln after the molten glaze freezes.
Most solids expand when heated and contract when cooled. Glaze and body are glued together so obviously they need to expand and contract the same amount during heating and cooling or something has to fail.
Typical glazes naturally tend to have higher expansion (and thus higher contraction) than bodies. The vast majority of mineral mixtures that melt the proper amount at pottery temperatures will be crazed on typical pottery bodies. Crazing is thus 'nature's norm'. That means that special attention is needed to produce the abnormal result of proper glaze fit.
Bodies high in silica expand more and thus craze less (and vice versa). A good demonstration is to calcine some ball clay (it is very high in silica) and mix raw and calcine 50:50 (it is too plastic and sticky if you don't do this), slurry and dewater to make a body, then make, glaze and fire a small bowl. The glaze will shell off on cooling in the kiln (this is the opposite of crazing, it is a size 6 glaze on a size 5 body). Now make a body of 50% feldspar and 50% plastic kaolin (eg. Tile 6) and glaze and fire. The glaze will craze badly on this. The lesson: The latter body has no quartz, quartz content is the major factor in increasing body thermal expansion. That means more quartz makes glazes craze less. Unfortunately higher thermal expansion is a bad thing for other reasons (it means more susceptibility to cracking due to sudden temperature changes during ware use).
Oddly, glazes high in silica expand less when heated, this is the opposite of what bodies do. This is because the fluxes dissolve the quartz particles and they become part of a silicate glaze structure. In glazes chemistry is the key issue, in bodies the issue is mineralogy. To demonstrate, mix nepheline syenite with about 5% bentonite and make a glaze and apply to a pot and fire at cone 10. You will find a severe case of crazing on any body. Now blend silica and nepheline 50:50 (with 5% bentonite). Crazing will be dramatically less. Add some boron (from a frit or BORAQ) and the extra melting will enable you to add more silica which will reduce and eliminate the crazing.
The moral is that you are in control if you want to be. Body and glaze formulation are the major factors in crazing. Give me a glaze and I can adjust a body's mineralogy to fit it. Give me a body and I can adjust a glazes chemistry to fit it. Thus if your glazes always seem to craze on a body it means either that the body has less silica than is right for you or your glazes lack silica (or employ too much high expansion soda/potash contributing materials like feldspar).
Industry typically uses bodies of much lower silica since they are capable of formulating glazes of low expansion to fit. The benefit is ware of low thermal expansion that can withstand thermal shock. Pottery porcelains, on the other hand, have plenty of quartz so less technical customers have fewer problems fitting their glazes. Actually, few potters fit anything, it often just a matter of chance that their glazes fit the bodies. Bodies trend toward certain expansions in different parts of North America and often when potters move across the continent they find their glazes don't fit bodies available in the new area. This is especially true where clays native to an area are employed.
I'm going to bash crazing twice more: Crazing is not just unsanitary, but it weakens pieces up to 400% according to our tests. Why? Each minute glaze crack wants to extend itself into the body when stresses are applied. Ceramic propagates cracks very well because of its brittle nature. If you have a smooth crack and defect free surface cracks have nowhere to start and ware is much stronger and more durable over time. But crazing weakens it and supplies thousands of started cracks just waiting to extend themselves. Time and continued use are the catalyst to eventual chipping, cracking or premature breakage. Craze lines are also the point of entry of water into porous bodies (almost all stoneware bodies have some porosity). This results in further weakening and waterlogged pieces that can become 'bacteria farms'.
So take this whole issue seriously. Examine your colored glazes very closely to see if they are crazing. Stress your ware using boiling and ice water to bring out delayed crazing. For more information, search for 'crazing' at digitalfire.com.
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