Maura on thu 10 mar 05
Hi there,
I'm a newbie to glazing. Reading a bit and learning as I go but I haven't
been able to solve this problem.
Found an amazingly beautiful glaze that I'd love to keep using. Problem?
Designed for ^10-^11 while our school fires only to ^9. I believe this may
be the reason most of my pieces have pinholes. I'm thinking the glaze
isn't fully matured and is caught in place. Longer soak time may help if
it were my kiln to control. Of course, if that isn't it, and you have an
another idea I'd really appreciate hearing from you. It's a blue rutile
glaze. I've read they can be tricky. Do I add a percentage or two, five or
ten, of silica? Frit?
Thanks for your help.
Clueless in the SF Bay area...
Maura
Ron Roy on fri 11 mar 05
Hi Maura,
I'm off to NCECA - be back about the 21st - send me the recipe them and
I'll lower it for you.
You may have some luch just taking out some clay - do a 5 part line blend
taking out 1% clay each step - watch out for running when you start taking
out clay.
RR
>Hi there,
>I'm a newbie to glazing. Reading a bit and learning as I go but I haven't
>been able to solve this problem.
>
>Found an amazingly beautiful glaze that I'd love to keep using. Problem?
>Designed for ^10-^11 while our school fires only to ^9. I believe this may
>be the reason most of my pieces have pinholes. I'm thinking the glaze
>isn't fully matured and is caught in place. Longer soak time may help if
>it were my kiln to control. Of course, if that isn't it, and you have an
>another idea I'd really appreciate hearing from you. It's a blue rutile
>glaze. I've read they can be tricky. Do I add a percentage or two, five or
>ten, of silica? Frit?
>
>Thanks for your help.
>
>Clueless in the SF Bay area...
>Maura
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Logan Oplinger on fri 11 mar 05
On Thu, 10 Mar 2005 12:08:51 -0500, Maura wrote:
>Hi there,
>I'm a newbie to glazing. Reading a bit and learning as I go but I haven't
>been able to solve this problem.
>
>Found an amazingly beautiful glaze that I'd love to keep using. Problem?
>Designed for ^10-^11 while our school fires only to ^9. I believe this may
>be the reason most of my pieces have pinholes......It's a blue rutile
>glaze. I've read they can be tricky. Do I add a percentage or two, five or
>ten, of silica? Frit?
>
>Thanks for your help.
>
>Clueless in the SF Bay area...
>Maura
Hello Maura,
Your glaze recipe can be adjusted to fire at a lower temperature, but
without knowing what the quantities of the different ingredients are in the
recipe, I can offer only a general approach to take. I am going to assume
you do not have a computer glaze calculation program, or access to one. If
you can provide the Clayart group with the recipe, then one of us who has
such a program (Sorry, I don't!) could run the calculations necessary for
you. In the past Ron Roy & John Hesselberth have assisted other potters
with adjustments to their glazes. If one of them catches your post, they
may offer to help.
It may not be appropriate to increase just one or two ingredients. It may
me necessary to increase all of the ingredients which contribute fluxes to
maintain their balance, or conversely, decrease the ingredients that supply
only silica and alumina or both, such as clay (kaolin) and silica. Some
materials such as the feldspars supply both fluxes and alumina & silica.
If your recipe contains kaolin and silica, then it may be possible to
decrease both of these. If you do not know how to do your own calculations
with pen and paper, then the easiest approach will be to do a series of
sample glaze tests, gradually decreasing proportionately the amount of
silica and kaolin in the recipe for each test sample. Now the math begins.
For example, if a 100 gram batch of ^10-11 glaze contains 30 grams of flint
and 20 grams of kaolin (50 grams total), then for six sample tests it may
be appropriate to decrease the total amount of flint/kaolin in increments
of 2 grams. The first test batch (the control batch) contains 50 grams of
silica/kaolin and 50 grams of the other ingredients, and the sixth test
batch contains only 40 grams of silica/kaolin (and 50 grams of the other
ingredients). The incremental decrease is as follows: 50-0=50, 50-2=48, 48-
2=46, 46-2=44, 44-2=42, 42-2=40. To make mixing the tests easier by doing
less weighings, it helps to thoroughly pre-mix the silica/kaolin, and the
other ingredients as separate batches. Then combine these when making up
each test. The total amount of 30/20 silica/kaolin needed to make up these
tests is 50+48+46+44+42+40=270 grams. This would require 162 grams of
silica and 108 grams of kaolin in the pre-mix batch. (The total amount of
other ingredients is 50 grams x 6=300 grams in the pre-mix batch). By
applying each of these sample glazes to a test tile and firing to ^9, it
would be possible to see what the effect is of gradually decreasing the
amount of silica/kaolin.
Test #1 50g Other ingredients, 50g silica/kaolin (This is the control).
Test #2 " " 48g "
Test #3 " " 46g "
Test #4 " " 44g "
Test #5 " " 42g "
Test #6 " " 40g "
It may not be clear above how I arrived at the amounts 162 grams for silica
and 108 grams for kaolin. The proportion 30g silica & 20g kaolin is the
same as saying 3 parts silica and 2 parts kaolin, or a total of 5 parts.
One part in 270 grams is 270g/5 = 54 grams. So 3 parts silica is 54 grams
x 3=162 grams. 2 parts kaolin is 54 grams x 2=108 grams.
Learning how to do glaze calculations helps a great deal in figuring out
how to adjust recipes to increase or decrease the maturing temperature,
substituting different ingredients, in altering the character of a glaze,
and in making up your own glazes from scratch.
Good luck with your work, I hope what I have provided here is of some help.
Logan Oplinger
Another Tropical Island
Tig Dupre on fri 11 mar 05
---------------------------<>------------------------
Found an amazingly beautiful glaze that I'd love to keep using. Problem?
Designed for ^10-^11 while our school fires only to ^9. I believe this may
be the reason most of my pieces have pinholes. I'm thinking the glaze
isn't fully matured and is caught in place. Longer soak time may help if
it were my kiln to control. Of course, if that isn't it, and you have an
another idea I'd really appreciate hearing from you. It's a blue rutile
glaze. I've read they can be tricky. Do I add a percentage or two, five or
ten, of silica? Frit?
---------------------------<>------------------------
Maura,
Without seeing the recipe, it's difficult to tell you what to adjust to lower the temperature. Generally, adding more flux (feldspar or nepheline syenite) will lower the temperature of maturation, but doing so changes the composition of the glaze and the results will vary from what you expect.
Yes, rutile blues are tricky. Therefore, you need to get some glaze calculation software and work out what you wnat to do before you do any testing. I, personally have been using GlazeChem as my main program for about four years, and have a huge database of glaze recipes, most of which I will never use again. But, using the software has saved me a lot of time and money which would have been spent in empirical experimentation. There are many other glaze calculation programs out there; some are even free. I suggest you investigate and buy one, or even two. It helps to have a cross-check sometimes.
Welcome to the wonderful world of glazes! I think there are potters who strive for good form, others who strive for good glaze, and still others who just like to play with fire. A very lucky few strive for all of it!
Good luck,
Tig Dupre
in Port Orchard, WA
June Perry on fri 11 mar 05
I suspect your pinholing is being caused by other factors, i.e. too fast
firing, stacking too closely, improper, fired too fast, too low, bisque etc.
You might want to try firing bisque slowly, not stack too closely, and don't
fire the glaze too fast, which only causes more boiling of the glazes,
causing those glaze eruptions; and then if you don't soak enough at the end to let
those bubbles settle down and smooth out, you can wind up with a lot of
pinholes. Overfiring I have found, can also cause those pinholes in some glazes
particularly those with zinc, high whiting, for example.
Often, cone 10-11 recipes are also good at lower temperatures. Many
published cone 10 glazes that I have examined, are have a silica alumina content
lower than is called for in a cone 10 glaze.
Many glazes have a wide firing range. So, I would try some of the other
fixes first. If you want to post the glaze or email me privately, I'd be happy to
run it through my glaze software and see if it is within cone 9 limits. I
suspect that it is.
Regards,
June Perry
_http://www.angelfire.com/art2/shambhalapottery/_
(http://www.angelfire.com/art2/shambhalapottery/)
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