Don Kopyscinski on fri 22 mar 96
Hi everyone,
Recently Edd Cox wrote:
>Tom Coleman turquoise.
Cust feld 57
Ball 11
Stront. 23
Whiting 10
Zinc 6
Copper carb 4
Rutile 4 Together, Turq. matte With flow. Watch your feet.
Any Ideas on why it flows and is also matte?<
I would like to take a crack at this one.
There are a few concepts that need to be understood.
*Firstly the reason for mattness or glossiness in a balanced glaze, is normally
due to the ratio of silica to alumina in that glaze (This excludes flux
saturates and mattness caused by crystalization, but I'm talking about a
balanced glaze in which these phenomenons would not normally occur). Using
"Insight" I calculated the base glaze. Here that ratio is 5.77 - 1, truely in
the matt range (comparing the formula (molecular formula) with long established
well documented limit formulas available in much of the literature).
*Secondly the fluidity of a glaze is due to the ratio of fluxes (and the
relative fluxing power of those chosen) to the silica/alumina component of the
glaze. Lower alumina and silica yields a higer percentage of fluxes. Expressed
another way the normal cone ten glaze contains aproximately 55-70% of silica (an
aproximation off the top of my head for illustration purposes). This one
contains 47.8% of silica. So the balance is made up of alumina and more fluxes
than is normal for this temperature range.
In examining this glaze these theories evidently hold true.
1-RO .31 - Al2O3 1.77 SiO2
(generically bundling the fluxes, they are all within limits)
Comparing this to normal limit formulas for cone 10, the silica component of the
glaze is below limits. So it's matt because of the ratio of silica to alumina
and fluid because of the high proportion of fluxes to the available alumina and
silica.
So.....
1-adjust the silica upward by adding flint bringing it to the low end of the
limits in this case 3.0.
2-increase the alumina to achieve the same compliment as the original glaze.
This is first done by increasing the ball clay and adjusting the silica
accordingly.
This yields:
36.89 Custer
26.86 Ball Clay
14.89 Strontium Carbonate
6.47 Whiting
3.88 Zinc Oxide
11.00 Flint
Once done, it is seen that there is too much ball clay and that the raw glaze
layer is likely to crack. A substitution of some calcined kaolin to reduce the
shrinkage of the raw glaze layer is in order, while keeping the ball clay
component at about 10% for good suspension and adhesion properties. The
resulting glaze was then rounded off where possible to neaten up the recipe a
bit and the coloring oxides added back in their original proportion.
Revised Coleman Turquoise
38.0 Custer
10.0 Ball Clay
15.5 Strontium Carbonate
6.5 Whiting
4.0 Zinc Oxide
14.0 Flint
12.0 Calcined Kaolin
4 Copper carb.
4 Rutile
this yeilds:
1-RO .52 -Al2O3 2.99 - SiO
The ratio of silica to alumina remains virtually the same (5.8 to 1), the ratio
of fluxes has not been touched, and the formula now falls within limits of a
stable cone 10 glaze. A line blend of the original glaze and this version should
yield an increasingly stiffer one with the same color response and physical
character as the original, but more stable. If possible choose the most
satisfactory one as close to the new version as you can. I would love to hear if
the theory holds true in your case.
Missing NCECA but putting on a birthday bash for my 5 year old son David this
weekend.
Party, party, party!!!
Don Kopyscinski
Bear Hills Pottery
Newtown, CT
I could never have done this quick fix in a timely mannor if it weren't for
Insight...thanks again Tony Hansen!
I highly recommend the book "The Magic of Fire 2", also written by Mr. Hansen.
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