Tom Yocky on wed 29 aug 01
I'm really new to the list too, and have very little
actual experience with the "art" of glazing, but I
have done a lot of reading lately. Here is my
understanding of what causes crazing and how to make
"any" glaze craze (or if crazing is not desired, how
to stop a glaze from crazing). Hopefully some of the
true experts out there will correct me if I am
incorrect in any way.
Crazing happens because both the clay body and the
glaze material (glass) expand and contract due to
heating and cooling. Crazing happens when the glaze
"shrinks" more than the clay vessel under it. The
glass (glaze) wants to continue to shrink but the clay
body is done shrinking and pushes back. If it pushes
back too much, the glass starts to crack and break.
Shivering is the opposite, when a clay shrinks more
than a glaze and actually pulls away from the glass
surrounding it. So the idea to get a glaze to craze
is to "force" it. I could spout off a recipe that I
know crazes in my ^6 electric kiln, but the exact same
recipe may not craze in yours because you and I may
use different clays. So the best method I believe is
to start with your favorite base glaze, and determine
it's expansion based on not the ingredients you put
into the glaze, but the oxides that make up the
ingredients. Oxides (such as SiO2...silica /flint
/quartz) have been scientifically tested and well
documented expansion rates. By figuring out just how
much percent wise of each of the oxides you have in
your favorite base glaze, you can determine it's
expansion using simple algebra...or even more simply,
using a computer program such as Glazechem 2.1. I
found this program at
http://www.dinoclay.com/software/glzchem.html and it
has proven it's self extremely useful. With a program
like Glazechem 2.1, you simply enter in the recipe for
your base glaze (500 grams of this, 225 grams of that,
and so on) The program then gives you an analysis of
the recipe, including it's expansion rate. To make
your glaze craze, you increase or decrease whatever it
takes to raise the expansion of the glaze. if your
baase glaze shows "Expansion: 76.5 x 10e-7 per degree
C" then increasing that to 83 will most likely be
enough. I used the program myslef to stop a glaze
from crazing, but the reverse is easy enough as well.
I learned a lot on this Oxide approach to developing
glazes from the Magic of Fire book (which is sold
online but I just read all the information on the web
page at http://digitalfire.com There is so much
information there that it can be overwhelming but so
maybe start with http://digitalfire.com/magic/toc.htm
This is the book chapter by chapter that I read and
enlightened me completely. Best of all it's free. I
am not associated with it in any way. Also don't
forget about the program glazechem
I hope this helps. It was all greek to me jsut a few
weeks ago but with a little reading I feel like I know
a tiny bit about it.
Take care,
Tom
from Klamath Falls Oregon
--- Dale Cochoy wrote:
> I signed on list about two weeks ago, lurked awhile
> before asking about
> crackle glaze recipes for cone 6 firing. I got one
> reply from Tom Buck
> thanks Tom) about addition of a few chemicals to
> any base white. Since then
> I stumbled on Mayco's new crackle glazes which come
> in about 6 colors. They
> can be fired to cone 6 according to book. White
> won't crackle ( they say)
> but other get a fine crackle and colors fade. Does
> anyone have any
> experience with these yet at cone 6 electric.
>
> I also just purchased my first gas kiln, an Olympic
> 28X27G. Now the
> decissions about where to set it. At first I thought
> in garage. Two
> negatives there, open flames in garage ( we never
> park cars there but there
> gas fumes from mower, etc.) and I would have to bend
> angle into vent to run
> out back of garage. Everyone says "no angles, must
> be straight unless using
> fan", and some recommend no fan due to heat
> ruining it quickly.
> Second Idea was to build deck behind garage, cover
> deck surface with patio
> blocks and place kiln there surrounded by steel
> shed. Run gas line from
> home.. Originally I'd thought wood shed to look
> better but cost is higher
> and steel seems safer. I could open doors to fire,
> close when not in use.
> Thought of venting through roof of shed. But, do I
> really need to vent
> through roof at all if inside shed outside that has
> open doors.? It is a top
> load and will require a costly hood vent swing-away
> system. Does anyone
> fire with no vents. Does everyone with vents stack
> them to above eaves ?
> What is min. distance recommended to walls for cone
> 8-10 firings? Many
> questions. I could use some advice or comments about
> things I've possibly
> not considered.
>
> Regards,
> Dale
>
>
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