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4. Food & Durability

What is the best kind of pottery for food and durability? 

Porcelaneous-Stoneware, with 'high-temperature' glazes, (with a moderate amount of glaze compression).  These are the most durable pots and the safest glazes for food.  The goal is to reconstruct  the STRONGEST POSSIBLE ROCK.  And since the glaze is the part first exposed to any kind of abuse, both chemical and physical, it is especially important to the strength and durability of the pot as well.   

For glazes that are to be used for food, descriptive properties are the most useful. Lead-free glazes, & High-temperature glazes, are the ones to look for.  

'High-temperature' glazes are the only defined category of glazes that are guaranteed not to contain lead.  They are defined as those having a melting temperature above the volatilization temperature of lead - about cone five or 2,185 degrees F.  This means lead would not intentionally be used, or be in the finished glaze (if accidentally put in).  Stoneware, Porcelain and their hybrids are typically  fired well above this point - usually cone 8- 10, or  2,300 - 2,400 degrees  F.

For strength, non-porous, vitrified clay bodies have more complete welds between particles.   Smoothness and homogeneity can also enhance  strength.  Excessive glassiness beyond the point of  non-porosity, however, can actually increase brittleness.

But this is only half of the story of strength; especially if the pots are glazed. 

Regardless of the degree of porosity or vitrification or excessive glassiness of  the clay body, if a pot is glazed, the stress between the fired clay body and the glaze is the dominant factor that effects strength/weakness of glazed pottery.  In pottery it is called 'Glaze fit'.  To 'fit' a glaze must be in moderate amount of compression.    Glaze compression can easily more than double the strength of an unglazed piece.  Conversely, a glaze that is in tension can cut the strength down more than half of the unglazed piece. 

So what is the 'Standard' for 'Functional' pottery? 

Porcelaneous-Stoneware, with 'high-temperature' glazes, where the glazes are in a moderate amount of compression, are the most durable, and the safest for food.

A pebble mill, like the one above is handy to control the particle size of the different rocky materials used in the clay and glazes.

Note:  There are a few, lower fired, porous, earthen-wares that do not use lead as a means to lower the melting temperatures of the glazes. And, if these glazes are well matched with the clays expansion rate i.e., in compression, they can be reasonably strong, though still vulnerable to edge chipping.  The best low-fire pottery can occasionally be stronger than the worst of the  high fire pottery.  This happens when a stoneware is internally fractured from incompatible coarse aggregate,  and the glazes are in tension instead of compression.