May Luk on wed 8 jan 03
Dear all;
I'm trying to study a vellum glaze that I got from school and I'd appreciate
your help.
It's just called vellum. I use it on terracotta fired at 1100 C
I put a white or pale green slip underneath, plus some copper oxide
decorations. This is not for use with food.
Lead Basilicate (Pb0 2 SiO2) __50
Borax Frit (Na2B407 10 H2O)__20
Zinc Oxide (Zn)_______________6
China Clay (Al2O3 2SiO2 2H2O)__7
Zirconium (Zr) _______________12
Titanium (Ti)_________________2.5
Tin Oxide (Sn) ________________3
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
This is a simplified definitions I got from books. I do this to help me
memorize the ingredient properties and to understand what it does in the
glaze. Please correct if you see fit:
Lead Basilicate-Flux
Borax Frit-Flux
Zinc Oxide-create metallic crystals on the surface of the glaze (together
with titanium)
China Clay-source of alumina and silica, matting agent when used in excess
in glaze
Zirconuium-Opacifier; gives milky white, not as intense as tin. Promotes
visual textures.
Titanium-Opacifier; for crystalline glaze, produces a creamy white colour in
it's pure form. Promotes visual textures.
Tin Oxide-Opacifier; give good quality white.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
In school, the glaze always gives good results. But I'm curious with the
glaze and I'm going to do some test with my own clay / RM and my own kiln.
I'm interested to see if I varie the ingredients, what would happened, etc.
What would be a logical way to do:
1-I would like to do a "Reviewing Glazes-using the grid method" type test.
but I'm not certain how to start. Which RM to tinker with? I would welcome
another suggested method that requires less than 35 grids.
2-If I want to make the glaze leadless, do I just subsitute Lead Basilicate
with Borax Frit? Any other adjustments do I need before the test?
Thank you in advance
May
London, UK
Lily Krakowski on wed 8 jan 03
Ok. Converting from a lead glaze to a non lead one is not that hard--but,
it is like "converting" high calorie foods to low calorie ones. To a great
extent one kids oneself. No sane person can be convinced for a moment that
a recipe using butter, cream, sugar will taste as lovely made with oil,
evaporated skim milk, and artificial sweetener. To put it another way: no
makeup will replace the lovely glow of a young skin; close, fool some,
but...
Lead gives a soft, gentle velvety look to glazes. With tin and titanium it
gives a wonderful "glow". I have never been able to replicate that, it is
always "close to" but never quite there. (Or why, despite the opinijon of
my much-betters, I reject the term "majolica" for non-lead containing
glazes.
I would suggest you make blends, eliminating the lead gradually, as you
raise the boral frit, and other fluxes. I would try whiting, dolomite,
nepheline syenite. I do not know how Brits feel about lithium but that
might help.
In other words I would do a straight line blend of the original glaze and an
identical mix that replaces the lead frit with neph. sy, a straight line
blend that replaces the lead frit with lithium, etc. I then would take the
best and most promising of those and mix again....Ultimately I will have
eliminated all lead and get something hopeful.
Now that we all are sensitive to passing glaze recipes on, this is from
Emmanuel Cooper's The Potter's Book of GLaze Recipes--an invaluable book.
He has (p 37) a turquoise glaze that contains boral frit (3134) 18%
zirconium and 8 rutile. That IS NOT THE WHOLE RECIPE but it should be
guideline to the tolerance for high zirconium and a good dose of titanium
Having said all that--good luck...Lettuce know...
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