Wednesday, September 9, 2009

Clearly we are getting somewhere




When I came out of school and started doing glaze test I did not know anything about testing glazes. I meant not one thing!
I would read a recipe, look at the color and mix it up and I would get yuck.
Then I would quit.
I would rather throw pots.
Now I know there are some factors in getting the glaze you want.
First off what clay body do you want this to fit on?
What atmosphere are you firing in?
What is the color of clay body?
How are you firing and what is the temperature range you are looking for.
With all this in mind I went after the clear glaze.
The clay body I am using is Highwater's Red Stone.
Firing to 2185-2190
I wanted a good melt which would work well with slips.
Before the studio fire I was testing and working with slips at cone 5.
The fire took all the slips with it.( darn fire!)
I have finally had some time, let's rephrase that.
I have made some time to work on slips again and I wanted a really good clear glaze.
I tried Ron's clear glaze and it just did not have quite the melt I wanted- close but not quite there.
Thanks Ron!

I looked at a very old Ceramic's Monthly, 1986, and found a clear there using Gerstley Borate. Good melt, but with that blue tinge which come with Gerstley. Maybe for some thing else....

I then moved out on the Internet to find a glaze for slips by Kate the Younger.

Kate the Younger Clear Glaze
Cone 6
Raw Material
Ferro Frit 3195
70%
EPK Kaolin
8%
Wollastonite
10%
Silica
12%
Total
100%
Add:
Bentonite
2%
And in digging through my notes and such from past workshops I came up with a clear glaze from Ron Myers.
The Kate Younger and Ron Myers have won out.
Both had a good melt with good shine on the clay body.
I will fire yet another group of test today.
Red stone, white slip, color slips and see what comes out.
I'll also post up the other recipes, but I am in the basement office and the recipes are out in the studio.
I have to get them back here to put in the computer file with some notes.
I do have one more clear I came across which we used many years back.
I think it might need testing too.
It has only three ingredients.
3134 frit and something, something.... I'll post it up as well.

4 comments:

mike said...

I don't know if you already do this, but using glaze calculation software can be a HUGE help. When you look at the unity formula for a glaze you can really see what it's made of. So, for example if you want more melt while keeping your alumina and silica within an acceptable range you can determine what type of boron frit to add. Stuff like that. Once you have a couple parameters for what makes a stable glaze it really isn't hard to get the hang of. Many cone 6 recipies floating around are not very stable and will leach out any oxides added to them.
A year or so ago I wrote a free website to allow people to store recipies and calculate the unity formula for your glazes. The site is http://www.glazecalculator.com/.
Also, you can post your recipes. Here is the info on the glaze you posted as an example:
http://www.glazecalculator.com/glazecalc/recipes/11705/view.aspx

-Mike

Patricia Griffin said...

But, but, but, it all requires so much patience (whine, whine) and all that sciencey stuff and words like percentages, calculations and formulas.

Linda Starr said...

Boy what a lot of work researching glazes, but hopefully in the end you'll get just the perfect one you can use for years. Hey my word verification is oldcar, hehe.

mike said...

Patricia,
I would say that it significantly lessens the amount of testing you have to do. And if you use software you don't need any formulas. There are two ways to work on a glaze:
1. Take a recipe from the internet and try it. You look at it and see if it looks like "good glass" (unless it's a matte glaze by design, then you don't really know if it's melted or not!) and if it doesn't you guess at what to add and keep repeating the cycle until it looks like you want. The problem with that (besides time) is that it gives you no reason to believe that it is a stable glaze. Are we really OK with putting out glazes that are leaching copper into people's tea/coffee/food?
2. Take a recipe and use software to look at unity. Is the silica high enough? Does it have enough alumina? If it doesn't you can either adjust it or rule it out from the start. If it isn't melting enough does it have enough boron (or other flux)? If I need more boron and add a frit what does that do to silica and alumina? I don't mean to imply that silica and alumina are all there is to look at, but you can pretty much know that if they are not in a reasonable range the glaze will not be stable.

You can take quite a few cone 6 glazes that people use and ruin them by simply leaving a lemon slice on them overnight (or a couple hours). Of course if it's a clear glaze or has only a tiny bit of iron in it, there is really nothing to leach out (though the glaze may fail or dull). But if it has cobalt or copper, you don't want your customers eating/drinking that. Obviously if you create indoor sculpture this problem isn't a concern.

It really doesn't take much time to learn a few parameters to judge whether a glaze should be stable or not. Ultimately, unity numbers aren't the whole story and you need to test also. But it is a huge shortcut if the goal is to provide long-lasting, non-poisonous glazes to our customers.

All of the above refers to cone 6. It is significantly easier to make a stable cone 10 glaze.

Linda,
my word was suato. Not nearly as cool!

-Mike