Dave Finkelnburg on tue 6 jan 04
Gary,
Good for you for trying out your own glaze making! Funny, I was
reviewing slip glazing in Birks just last night. Lovely book, isn't it!
First, there is nothing to prevent your glaze from working well on food
ware. So far all the ingredients are safe for people, pets and other
important organisms. :-)
200 to 500 grams of dry glaze ingredients is a good amount for a sound
glaze test. It depends on the accuracy of your scale, and how careful you
are in your testing.
Yes, you are on the right track...adding some of your white clay will
help with the crazing, because it will have a lower expansion value than
this glaze. However, that will also add alumina, and this glaze already has
an awful lot of alumina for a cone 6 glaze. Silica is more commonly added
to lower glaze expansion. You will need a bunch with this glaze, and it
also has plenty of silica already. May I suggest an alternative?
In my biased opinion you have picked a somewhat unfortunate glaze. It
will craze on virtually any stoneware because it has so much sodium in it,
and sodium is a very high expansion flux. However, this glaze needs sodium
to help it melt early in the firing so the heavy alumina content (which
makes the glaze viscous) will not prevent the glaze from flowing out
smoothly on the ware.
I strongly suggest you just abandon this glaze and go to a sounder base
glaze, like Tony Hansen's 5x5, as a starter. See:
http://www.digitalfire.ab.ca/cermat/education/114.html
With that base if you want to try Neph rather than Custer Spar to have a
little more sodium in the flux column, you can test that. Even that glaze
may tend to craze, though, and on a white body you may need a lower, rather
than higher, expansion glaze.
Besides Clayart's archives and other online sources, there are some
better books available as far as getting started with glazing. One is
"Mastering Cone 6 Glazes," by Hesselberth and Roy. It is not a collection
of glaze recipes, although it has some, but more of a source of things
helpful to understand about the fundamentals of glazes. It's available from
a lot of clay suppliers and at http://www.masteringglazes.com/
Good glazing!
Dave Finkelnburg in Idaho
----- Original Message -----
From: "Gary Harvey"
Sent: Tuesday, January 06, 2004 8:50 AM
> I am new to glaze making. I tested a recipie for glaze 6 clear that I
found
> in Tony Birks "The complete Potter's commanion" Johannes Peters Glaze
> consist of 72 nepheline syenate, 9 calcium borate, 6 white stoneware clay,
> and 7 talc. I used gillespie borate for the calcium borate and OM 4 for
the
> white stoneware clay. The glaze crazed like crazy. (I will send a photo
to
> anyone that is interested) Can I use the white clay that is my normal clay
> and do any of you think that will fix the crazing problem? I want to use
> this on foodware. AND what amount of Glaze should I make up as a test
amount?
| |
|
