ELIZABETH JACOBS on sat 13 mar 99
I need some help with a cone six glaze recipe fired in an oxidation
environment. When I did a 200 gram test, I got beautiful results. When I
made a 5000 gram batch, my results were very different. The first tests
gave a satin matt finish with subtle specks of colors. The large batch
was much shinier and did not have the depth and variation of the tests.
I used new bags of F-4 kona feldspar and red art clay for the large
batch. Here are the recipes for these two glazes.
BASE CONE 6
kona F-4 50
Gerstley Borate 5
Whiting 15
Zinc 5
Flint 5
Kentucky Ball Clay 10
Red Art Clay 10
PUSSYWILLOW
Rutile Dark 4
Black Nickel 2
BLUEBERRY HILL
Black Nickel 2
Cobalt Oxide 1
Bentonite 2
The pussywillow test had a warm dark tan color with specks of deep
brown and a matt finish. The large batch was a medium shiny brown
without any specks.
The blue berry hill test was a blue-gray with specks of blue and brown
with a matt finish. The large batch had some areas of this color but was
mostly a shiny dark blue.
I did some placement tests and noticed that it was a bit shinier towards
the top.
If anyone has any suggestions of how I can match the 1st tests or a
site/ person who could help me figure out what is happening.
Thanks,
Elizabeth
David Hewitt on tue 16 mar 99
Hello Elizabeth,
I don't know if you have had any off list replies, but I haven't seen
any on the Clayart digest. Your problem is, however, one which I expect
most potters have experienced at one time or another. That doesn't mean
that I can give you a clear answer, but it does mean that I think the
subject is important and worth ventilating in this discussion group.
I can think of a few possible reasons for the difference that you have
found between your test glaze and the bulk mix. I am assuming that the
individual raw materials are from the same supply and hence this is not
a possible cause of the problem.
1. 200g is a small amount and hence the accuracy of your weighing
equipment is important. The 5000g mix would obviously be more accurate
unless an actual mistake was made.
So was one incorrect?
2. The thickness of application between test and bulk has been
different. For example, a thinly applied shiny glaze can become a satin
matt when applied more thickly. For this reason double dipping part of a
test tile can help to give an indication of how sensitive a glaze may be
to thickness of application.
3. A test tile has a limited surface area and angle. A pot has an inside
and outside surface at a variety of angles and this can have its affect
on how the glaze performs.
4. If there was a difference in the firing this obviously would have
made a difference, but I presume that you did not intentionally make
such a difference. Was the test done in a different kiln?
Others may be able to add to this list.
What then can you do? Considering the points above, and as you have 5kg
of the glaze, I would try applying the glaze as thickly as possible and
see what difference this makes. If you have very good weighing equipment
I would also try another small mix and test this at the same time. If
the new small mix gives you the results you want then try a slightly
bigger quantity on an actual pot. If this is OK then suspect your 5kg
mix. If none of this works suspect the recipe.
Looking at the glaze analysis I find it is fairly high in CaO and low in
MgO. My experience is that if you want a satin matt it would be better
if it were the other way round. Reduce the Whiting to 5% and add 10%
Magnesium Carbonate. It is within general limit formula for cone 6 but a
bit high on expansion.
David
In message , ELIZABETH JACOBS writes
>----------------------------Original message----------------------------
>I need some help with a cone six glaze recipe fired in an oxidation
>environment. When I did a 200 gram test, I got beautiful results. When I
>made a 5000 gram batch, my results were very different. The first tests
>gave a satin matt finish with subtle specks of colors. The large batch
>was much shinier and did not have the depth and variation of the tests.
>I used new bags of F-4 kona feldspar and red art clay for the large
>batch. Here are the recipes for these two glazes.
>
>BASE CONE 6
> kona F-4 50
>Gerstley Borate 5
>Whiting 15
>Zinc 5
>Flint 5
>Kentucky Ball Clay 10
>Red Art Clay 10
>
>PUSSYWILLOW
>
>Rutile Dark 4
>Black Nickel 2
>
>BLUEBERRY HILL
>
>Black Nickel 2
>Cobalt Oxide 1
>Bentonite 2
>
>
> The pussywillow test had a warm dark tan color with specks of deep
>brown and a matt finish. The large batch was a medium shiny brown
>without any specks.
>
>The blue berry hill test was a blue-gray with specks of blue and brown
>with a matt finish. The large batch had some areas of this color but was
>mostly a shiny dark blue.
>
>I did some placement tests and noticed that it was a bit shinier towards
>the top.
>
>If anyone has any suggestions of how I can match the 1st tests or a
>site/ person who could help me figure out what is happening.
> Thanks,
> Elizabeth
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
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