Dave,
Good explanation of glaze fit.
'Crazing" is a defect and "crackle" is an effect. Semantics eh? Both physically weaken the piece some. Visually it can be quite attractive.
Same thing...technically... but some people like the look of crackle glazes....makes things look "old" with that pattern of lines and veins. (like an old potter's hands...)
You can stain the crackle with tea...or ink to make it show more. Rubbing with a glaze stain and refiring can accentuate the crackle in some wonderful ways. Crackle in a raku white glaze that has been fumed nicely can look quite interesting too.
IMHO.
Ric Swenson 770 923-8816, Atlanta
>From: Dave Finkelnburg <dfwengr@YAHOO.COM>
>Reply-To: Clayart <CLAYART@LSV.CERAMICS.ORG>
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Re: Pinging Pots
>Date: Mon, 27 Jun 2005 19:02:45 -0700
>
>Murry,
> IF you look at your pots with a magnifying glass, you will see crazing, and that's what you hear. The pinging is normal only if you consider crazing "normal." People here will argue about that. It is a fact that crazed ware is about 20 to 25% as strong as uncrazed ware. It's also a fact that there are pots around today with a ton of crazing in the glaze and the pots were made 2,000 years ago!
> As I suspect you know, when the fired body cools the glaze is shrinking more than the body. I imagine your clay body is underfired. You need
to measure the ability of a fired, unglazed test tile to absorb water. It should absorb less than 3% of its weight. There is an ASTM procedure for making the test. If you look in the Clayart archives you will find a version of it explained in detail. Search on absorption.
> If you fire the body so it's properly vitrified, you will REDUCE (not yelling here) the tendency for crazing. I emphasize the word reduce because it won't eliminate crazing if the thermal expansion mismatch is too great.
> Here's what happens. Firing higher melts more quartz (the highest expansion raw material we use) and creates more glass phase (about the lowest expansion material in our fired clay body. The net effect is the body has a lower coefficient of thermal expansion.
> To say any clay body has a firing range from cone 6 to 10 is, frankly, sadly, highly misleading. Your body is more likely
a cone 9-10 body. At cones 6, 7 or 8 it's underfired. However, for a lot of work you can use underfired clay. It's porous and that may be an advantage...say for African violet pots, or for thermal shock resistance...but it creates glaze fit problems and may even leak water if you use the pot for a vase. :-(
> You can lower the CTE of the glaze by adding some silica or clay. That will change the glaze somewhat, though. How much depends on how much you add, so you can only tell by testing.
> This is very brief and doesn't answer all the questions I am sure you have, but I hope it scratches the surface of the subject for you.
> Good potting!
> Dave Finkelnburg
>
>Murry Gans <mgans@MAIL.HOCKADAY.ORG> wrote:
>I am a new potter and installed a small Paragon test kiln at home about a
>month ago so I can now fire pots at home. I am using a
cone 10 stoneware
>from Trinity Ceramics, rated from cone 6 to cone 10, and Spectrum glazes.
>The witness cones show that I am reaching cone 5.
>
>Here is the question -- some of the bigger pieces ping for several hours
>after I bring them in from the kiln. I am letting the kiln cool until the
>pots are only warm to the touch, about twice the firing time, before I
>take them out of the kiln. I cannot see any crazing on the pots as a
>result of the pinging. Is this pinging normal? Is it going to hurt
>anything?
>
>
>
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