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Sputty

Strength of claybodies

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I'm starting this new thread with a post from Sputty that was originally in the thread 04 for dinnerware  Think it's a jumping off point for further discussion without the risk of the 04 Dinnerware thread going too far sideways.

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Toughness = tested by modulus of rupture. Edit:  And I think this is tested by the slow loading they talk about.  “Strength”  = hardness—and the impact tests they do.  Chipping is a function of low hardness, cracking via thermal stress is a function of low toughness.  Their impact tests are different and geared to measure different things.  

It’s why cultures that use open flame cooking with ceramic prefer low fired earthenware (and by low, I mean like cone 017, low enough that it doesn’t ring).  Also why hard paste porcelain was the holy grail of functional serving ware.

The terms are a bit confusing, but the general take away is the same as the Arbuckle thread linked (though there’s a more formal discussion of this somewhere that she makes).  Low fired is tougher, high fired harder.  Terracotta for the cazuela, porcelain for the serving dish.

At least, that’s my interpretation and experience.

Edited by Tyler Miller

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You also have to consider design when it comes to the longevity of pottery. Certain elements of pottery forms are more likely to chip than others, regardless of the type of clay it's made from. A foot that flares out is more likely to chip than a straight foot, a flared rim is more likely to get chipped than a straight rim, narrow or sharp edges chip easier than wider, rounded edges, etc. If you know the limitations of your clay body, you can design with that in mind, or select the best body for a particular design.

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Different but the same.

Friend artist etched designs into rock.

Soft rock hard to do as the rock absorbs the force he exerts.

Hard rock excellent results

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It would be interesting to see studies done with pots that have been in daily use for a number of years. Mature, not over or under-fired pots with well fitting glazes. If you follow the logic of a well fitting glaze strengthening a pot whereas a crazed one weakens it and combine that with the issue that earthenware is porous which often leads to delayed crazing (from water absorption) then it would seem logical that a vitrified body with a well fitting glaze is going to be stronger than one that has been subject to years of moisture related crazing. Like someone mentioned in another thread they both will break if dropped so maybe this is a moot point. Perhaps around cone 2 is where we should be looking, add enough flux to vitrify the body without having to use as much as you would need to make a low fire fritware, but easier on the firing costs than mid or high fire. 

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Just saw this today:

From The New York Times:

This Porcelain Is Tougher Than It Looks

The Hong Kong jeweler Wallace Chan spent seven years developing a type of porcelain that is five times harder than steel, and he’s more than ready to demonstrate it.

https://www.nytimes.com/2018/07/20/fashion/jewelry-porcelain-wallace-chan.html
 

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