[Clayart] (no subject)

Paul Gerhold via Clayart clayart at lists.clayartworld.com
Fri Jul 31 16:44:12 EDT 2015

  What I was using thickness of the glaze for was  one of the components in calculating the surface area of a glaze only bond.  To restate if you glue a bisque foot to a bisqued surface and then glaze, the total surface area of the bond Is the circumference of the foot times the glaze thickness. This is slightly simplified from the actual mathematical formula, but close enough for illustration.

Now,if you could actually have two perfectly flat surfaces and totally glaze the contact surface of the glaze and other surface you might have a half way decent bond strength or at least have a decent surface area for the bond.

And as for your point about the reason  production potters don't usually glaze to attach handles I would ask if you,or any other production potters use glaze only to attach anything to functional ware as a part of normal production.  I am betting that they don't because they know a clay plus glaze bond is significantly stronger than a glaze bond alone which was the original start of the discussion.

And your point about glaze removing clay forgets that if you try to remove a properly bonded clay join it will remove clay as well.


Sent from my iPa

On Jul 31, 2015, at 11:20 AM, Jim Brown via Clayart <clayart at lists.clayartworld.com> wrote:

> "Of course glaze may take off some clay but that is no proof of a really
> strong bond for something like feet.  The surface area of the bonding area
> is overwhelmingly  critical.  Let's take a foot one inch in diameter with a
> glaze thickness of say 1/64 of an inch.  Total area of glaze bond somewhat
> simplified is  pi X d X thickness of glaze which is about  .05  square
> inches.  A clay bond would have an area of pi X  r squared or .78 square
> inches about 16 times the surface area of a glaze bond.
> And hardness of glaze has absolutly nothing to do with the strength of he
> bond between glaze and clay.  Some of the hardest materials ( Mohr scale)
> shatter dramatically on impact.  There is a reason production potters join
> handles and feet before glazing and it has to do with an instinctive and
> experiential understanding that it makes a much stronger join.
> Of course if you actually want to test make a series of mugs with the
> handles joined with glaze alone and then sell them.  But, I would retain a
> good lawyer first.: - Paul
> Ah, what?
> First, I am certainly no math expert but what does SURFACE AREA have to do
> with the thickness of glaze or anything?  It's SURFACE AREA and the surface
> area of anything 1 inch is the same - clay, glaze or ?
> Second, glaze will usually shatter more than clay because it is, after all,
> glass.  Clay that has been taken to the melting point will also shatter
> quite well.  This might be a problem is one had a VERY thick layer of glaze
> but not what would normally be used to stick the foot to the pot.
> Third, the reason potters - production or otherwise - don't normally use
> glaze to attach handle has to do with the fact that since the glaze melts
> in the burning the handle would therefore either fall off the pot or slide
> to another position.
> In closing, I stand by my original post - if when attempting to remove
> glaze from a pot, or a pot stuck to a shelf, takes part of the clay off the
> pot then it is very clear that the glaze is stronger than the clay. - jb
> *                       JIM BROWN*
> *          Brown's Pottery Hiddenite*
> *  "Making handmade pottery . . . *
> *                                                . . . since the 1700's"  *
>                   *   386 479-4515*
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