tmartens on fri 25 jun 99
Hello all I got the recipe below off Clayart a while ago. The
colour is very pretty but the original batch I made was very
matte at ^6, decided I would keep that for some things but
would also like a gloss version.
I lowered the clay content and upped the Neph Sye and
whiting. This next batch was dry, then lowered the clay a bit
more.......it got drier. I am puzzled, I might not have ended
with a balanced glaze, but should it not have become glossier?
I am trying, ( LOL sometimes very trying) but that glaze
dragon still knocks me flat on my face
every now and then, not that I won't get up and fight with the
damn thing again, but a knight in shining armour would be nice
round about now.
Toni M in a delisciously chilly Durban South Africa
Forest Green ^6 (works at ^ 10
Dolomite 19
Whiting 3
Neph Sy 35
Ball Clay 24
Silica 19
100
Chrome Ox 2
Cobalt Carb 1
Michael Banks on sun 27 jun 99
Hi Toni,
Chris Schafale and Mike Bailey are quite right in directing your attention
to the high alkaline earth content of this glaze as the source of the
problem.
The current glaze has about 0.47 mol proportions of CaO, 0.28 MgO in the
unity formula and only 2.85 silica. This is well within the region which
produces lime-magnesia matts. These glazes are not underfluxed but achieve
their matt surfaces by microcrystal growth on cooling. The effect is
similar to that observed in liquid honey which has been left on the shelf
too long. It starts out clear and glossy but over time, sugar crystals
precipitate all through it, matting the surface.
By reducing the ball clay content or adding extra neph sy and dolomite you
are not addressing the problem which is a high CaO+MgO:silica ratio. You
need to add more glass-former to make it glossy, e.g: silica, silica-bearing
frit or borosilicate frit. Gerstley borate is an inexpensive option and
soda feldspar will eventually work too, but you need to add a lot of it to
counter the high CaO+MgO.
The myth that all matt glazes are under-mature and under-fluxed is
widespread.
Michael Banks,
Nelson,
New Zealand
>----------------------------Original message----------------------------
>Hello all I got the recipe below off Clayart a while ago. The
>colour is very pretty but the original batch I made was very
>matte at ^6, decided I would keep that for some things but
>would also like a gloss version.
>I lowered the clay content and upped the Neph Sye and
>whiting. This next batch was dry, then lowered the clay a bit
>more.......it got drier. I am puzzled, I might not have ended
>with a balanced glaze, but should it not have become glossier?
>I am trying, ( LOL sometimes very trying) but that glaze
>dragon still knocks me flat on my face
>every now and then, not that I won't get up and fight with the
>damn thing again, but a knight in shining armour would be nice
>round about now.
>Toni M in a delisciously chilly Durban South Africa
>Forest Green ^6 (works at ^ 10
>Dolomite 19
>Whiting 3
>Neph Sy 35
>Ball Clay 24
>Silica 19
100
>Chrome Ox 2
>Cobalt Carb 1
Michael Banks on sun 27 jun 99
Hi Toni,
Chris Schafale and Mike Bailey are quite right in directing your attention
to the high alkaline earth content of this glaze as the source of the
problem.
The current glaze has about 0.47 mol proportions of CaO, 0.28 MgO in the
unity formula and only 2.85 silica. This is well within the region which
produces lime-magnesia matts. These glazes are not underfluxed but achieve
their matt surfaces by microcrystal growth on cooling. The effect is
similar to that observed in liquid honey which has been left on the shelf
too long. It starts out clear and glossy but over time, sugar crystals
precipitate all through it, matting the surface.
By reducing the ball clay content or adding extra neph sy and dolomite you
are not addressing the problem which is a high CaO+MgO:silica ratio. You
need to add more glass-former to make it glossy, e.g: silica, silica-bearing
frit or borosilicate frit. Gerstley borate is an inexpensive option and
soda feldspar will eventually work too, but you need to add a lot of it to
counter the high CaO+MgO.
The myth that all matt glazes are under-mature and under-fluxed is
widespread.
Michael Banks,
Nelson,
New Zealand
>----------------------------Original message----------------------------
>Hello all I got the recipe below off Clayart a while ago. The
>colour is very pretty but the original batch I made was very
>matte at ^6, decided I would keep that for some things but
>would also like a gloss version.
>I lowered the clay content and upped the Neph Sye and
>whiting. This next batch was dry, then lowered the clay a bit
>more.......it got drier. I am puzzled, I might not have ended
>with a balanced glaze, but should it not have become glossier?
>I am trying, ( LOL sometimes very trying) but that glaze
>dragon still knocks me flat on my face
>every now and then, not that I won't get up and fight with the
>damn thing again, but a knight in shining armour would be nice
>round about now.
>Toni M in a delisciously chilly Durban South Africa
>Forest Green ^6 (works at ^ 10
>Dolomite 19
>Whiting 3
>Neph Sy 35
>Ball Clay 24
>Silica 19
100
>Chrome Ox 2
>Cobalt Carb 1
Ray Aldridge on mon 28 jun 99
At 11:50 AM 6/27/99 EDT, you wrote:
>----------------------------Original message----------------------------
>Hi Toni,
>
>Chris Schafale and Mike Bailey are quite right in directing your attention
>to the high alkaline earth content of this glaze as the source of the
>problem.
>
>The current glaze has about 0.47 mol proportions of CaO, 0.28 MgO in the
>unity formula and only 2.85 silica. This is well within the region which
>produces lime-magnesia matts. These glazes are not underfluxed but achieve
>their matt surfaces by microcrystal growth on cooling. The effect is
>similar to that observed in liquid honey which has been left on the shelf
>too long. It starts out clear and glossy but over time, sugar crystals
>precipitate all through it, matting the surface.
>
>By reducing the ball clay content or adding extra neph sy and dolomite you
>are not addressing the problem which is a high CaO+MgO:silica ratio. You
>need to add more glass-former to make it glossy, e.g: silica, silica-bearing
>frit or borosilicate frit. Gerstley borate is an inexpensive option and
>soda feldspar will eventually work too, but you need to add a lot of it to
>counter the high CaO+MgO.
Another possibility, somewhat less practical, for making recrystallized
mattes glossier, is to force cool the kiln through the period critical to
forming the crystals. I like to add titania to some glazes to add a degree
of visual and tactile texture, and if I test these glazes in a fast-cooling
kiln, they will be flawlessly glossy. If I "fire down" so that the glaze
goes through an extended cooling during the critical phase, then the glaze
will develop matteness in certain areas.
I haven't done this with lime-magnesia mattes, so I can't say if it would
work the same way. Perhaps the precipitation of these crystals is too
abrupt to be forestalled by forced cooling.
Ray
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