reactive (crater) glazes help !!
potty
could anyone help with a little background information, basally
what ingredients do what in the kiln to form the crater effect?
I was going to write more on the aesthetic effect, but need to know
what does what as not to sound to thick. also when i set up on my own
i want to persue this type of glaze for my own work, the school frowns upon
things that they dont know in their kilns.
any help or pointing in the right direction would save my life, i was
initaly
going to reference Lucy Rea !
many thanks. doug.
Brad Panek
From what I have read...
During firing a mature glaze bubbles and then smooths out when it
reaches maturity. A crater glaze is one which the bubbles are still
active when the firing is cooling, so that the bubbles still remain in
the glaze.
I had a Red glaze do this on some pots a long time ago, however it was
not the desired effect.
Steve Mills
coarse Silicon Carbide, tho' quite how much in what I'm not at all sure.
Hopefully someone else in the group will know more about it.
Steve
Bath
UK
In article <9eada5cc.01122...@posting.google.com>, Brad Panek
<btp...@msn.com> writes
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Steve Mills
Bath
UK
Dewitt
Chappell (The Potter's Complete Book of Clay and Glazes) has a section
on crater glazes. Some use fluoride compounds (cryolite, calcium
fluoride, and/or sodium silicofluoride), some magnesium carbonate, and
others use silicon carbide.
deg
Natalie Winter
<steve...@mudslinger.demon.co.uk> writes
>If my memory serves me right the material you need for this effect is
>coarse Silicon Carbide, tho' quite how much in what I'm not at all sure.
>Hopefully someone else in the group will know more about it.
You're quite right, Steve.
It'll work in any high-fire glaze (I don't know whether earthenware
firings would get hot enough to make the SiC decompose, somehow I doubt
it), and you only need a little. Start at about one percent and adjust
based on the effect you want. Personally I've never much liked the
effect, but I know others who delight in cratery surfaces.
The solvent action of any molten glaze of sufficient temperature will
attack the SiC, causing the production of free carbon which oxidises to
CO2 and bubbles the glaze. This also causes local reducing conditions
which can affect glazes in bizarre ways (in fact very finely ground SiC
is sometimes used to produce local reduction with a minimum of
cratering).
Myself, I prefer reactive glazes/underglazes that interact to produce
bubbly effects, whorls and splatters. The principle is the same, but the
bubbles are allowed to heal over and the result is a smooth but
patterned glaze. My recipes for these use carbonates that decompose at a
lower temp than SiC and hence give the glaze a chance to mix and heal.
Incidentally certain other things work to give crater effects of less
drastic proportions than SiC -- I've used fluorite (CaF2) to good effect
in low-fire glazes.
hope this helps
Nali.
Natalie Winter
The Dawnmist Website:
http://www.dawnmist.demon.co.uk
Annemarie
there was an article about!!! :o)
Here it is
Great Craters
A Glaze Project by Haley Reid, Otago Polytechnic, School of Art
My interest in crater glazes first arose from seeing the work of Lucie Rie.
Rie used various methods to get textured surfaces. She often used layers of
slips and glazes. One ingredient she used to get cratered surfaces was
silicon carbide. Information on her glazes and techniques is very limited
and I searched widely through magazines and books to establish additional
information.
Silicon carbide is a compound of silicon and carbon and is used as an
electrical semi conductor, Kiln shelves and elements and grinding and
cutting disks and papers. Sometimes it is used to give artificial reduction
electric glazes.
Silicon carbide when heated in a glaze remains unchanged till 1000 degrees
C. Above this temerature it is attacked by the molten glaze with the oxygen
taken from the glaze, the silica joining the glaze melt. The carbon gasses
form bubbles in the glaze that rise to the surface bursting and escaping.
This is the key to creating crater glazes.
I used Abbots White and Southstone (NZ clays) in my experiments and
established basic slip and glaze recipes. These were fired in electric
kilns between 1240 - 1260 C. The killns were fired at 100 degrees per hour
with a 10 minute soak at top temerature. I put a layer of slip and a
brushed layer of glaze on before bisue firing and then another layer of
glaze before glaze firing. Many different colourants were tried and reacted
differently. Copper gave various blues and reds, Nickel huge blisters, no
craters unless broken later and a matt green. Ilmenite black edges and
cream glaze and an extra 5 percent silicon carbide an even small cratered
surface. All reacted better with the slip under. Care should be taken when
touching them out of the kiln as not all bubbles will have burst and careful
working of the surface with sandpaper or grinder can resolve the finished
surface. Have fun.
Slip
Ball clay 60
China Clay 40
Silicon Carbide 3
Baze glaze
Neph Syenite 60
China Clay 11
Silica 11
Barium Carbonate 18
Silicon Carbide 2.5
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