Lizacat29@AOL.COM on tue 11 jul 00
Ok..I've been raku firing with my local pottery supply's already mixed glazes
(a copper sand, white crackle, yukio's rainbow, & lithium carb) for about a
year. Feeling restive, I want to strike out on my own and mix glazes, I just
don't quite know how to start. The books just don't seem to get as
elementary as I need to be at this point. It's the numbers..they have me
I look at recipes and I see percentages (obviously the ones that add up to
100) and then "batch". Those numbers are the ones I don't understand. Some
add up to 100 just like the percentages, and others add up to odd numbers
like 8.63 or 120. How does that work? And sometimes, I have no idea what
the numbers mean. For example, some books say at the beginning of a large
list of recipes: " some are by volume and some are by weight" but I can't
always tell which is which.
I understand volume is like teaspoons, measuring cups etc. and weight is
grams etc. But, which does "batch" refer to? And do I have to invest in a
gram scale to make my own glazes or can I just measure? I assume that for
consistency, one must be quite accurate, so is weight better? Gram scales
seem expensive, what is the best kind to buy, can you get them cheap? I also
have read about conversion programs to convert recipes from one kind of
number to the other but I first need to know what all the numbers mean in
terms of what I do with them once I have them, right?
How big a batch would I want to make up to start?
Another question..Gerstley borate seems to have disappeared off the face of
the earth..Why is that? Is it a natural mineral that is all gone? And I
read the substitution articles with interest, but so far, no one seems to
have a great deal of success. So, would you recommend, since I'm just
beginning, not even trying any recipes with gerstley borate (assuming I could
even purchase it) because eventually it won't be around?
Thanks in advance for your help!
KYancey on tue 11 jul 00
The first book I learned on was Clay and Glazes for the Potter by Daniel Rhodes. I
must have read this book front to cover about a dozen times. I went over it with a
highlighter, kept a pad and pencil handy for notes and just took all the time in
the world to make sure I understood what glaze mixing was all about. Most of your
questions will be answered in this book After having fun mixing available recipies
for a while, I knuckled down to learn how to write my own glazes formulas. I'm not
a fast learner so I just took my time and practiced on my note pad as I read
through the book. It is my opinion that this is essential to mixing your own glazes
since your next venture should be adjusting and line blending your newly developed
formulas. This may sound a little intimidating at first, so again I stress. Take
the time to carefully read and re-read the tough parts as well as practice writing
formula. The rewards will be well worth it.
If I get figures like 8.63, I just round them out using the usual math rules. Or
you can multiply the figure by 10 to move the decimal point to the right one digit.
Of coarse you get a larger quantity of glaze.
If you have any pull with the police department, you might be able to get a gram
scale free. They have way too many from drug busts.
My suggestion is to stay away from Gerstley Borate and try some of the advertised
substitutions. This will probably require an addition of bentonite to the recipie
somewhere in the 3 to 5 percent of the batch. This is why you will want to sit
down and cover the written material. What is bentonite? and why do you need to add
it to a GB substitute when the ad says it can be substituted 1 for 1.
www.kickwheel.com gives the explanation.
Anyway, other clayart members will have tons of suggestions for you, so enjoy. This
is fun stuff!
Paul Lewing on wed 12 jul 00
This question about adding up to 100 and whether that's weight or
percentage is one of the most common questions I get in workshops. In
fact, I suggested it as a topic to Pete Pinnell for his Clay Times
column. I don't know if he's going to use it or not, but I've been
thinking of writing an article for Pottery Making Illustrated on the
Basically, there's a convention about how glaze recipes are written.
It's that the ingredients for the base (the glaze itself with no
colorants) add up to 100, and that is a weight measurement. It can be
any weight measurement- grams, ounces, tons, whatever- but not a volume
measurement like cups or gallons. Glazes are usually expressed in
weight measurements because it's much more accurate than volume
measurement. Raku glazes are about the only glazes that are ever
expressed in volumes, and that's because they are typically very simple
mixtures of two or three ingredients, and the temperature you're firing
to is inexact, so the measurement can be inexact. There are formulas
for converting volumes to weight based on the chemical composition of
whatever you're dealing with, but it's more complicated chemistry and
math than any of us need to do. So you want to use weight measurement.
And you do want to deal in grams because the whole ounces and pounds
system is ridiculous for converting from small batches to large ones.
So a gram scale is a good investment.
The reasons base glazes add up to 100 are two. First, 100 grams is a
good manageable size for a glaze test. And the base adds up to 100 with
the colorants listed separately so you can compare different glazes. If
they added up to 100 with the colorants included in the 100, a glaze
with 1% colorant wouldn't look like the same glaze with 12% colorant
added. So it's just for ease of comparison.
Are those numbers grams or percents? Well, if it adds up to 100, they
are always percent by definition. If you're making 100 grams then those
numbers are both grams and percents. If you're amking any size other
than 100, then they are still percentages but the figure for each
ingredient will be the percent value times whatever multiple of 100
you're making for a batch size. Make sense?
So what's a batch size? That's how much of the glaze you're making up
at a time. If you're making a 100g batch, the numbers are grams and
percent of a 100g batch. If you're making a 5000g batch, which is a
good size for a 5-gallon bucket, and a recipe that adds up to 100 calls
for 20 talc, you put in 1000g of talc, because a 5000g batch is 50 times
as big as a 100g batch and 50 times 20 is 1000. Got it? Pretty easy,
right? So how big a batch you make depends on what you're doing with
it. If you're dipping pots, a 10,000g batch might be about right. If
you make glazes in a garbage can, 50,000g would be better. If you're
brushing glazes on, 500g or 1000g would be enough.
I hope this helps.
Paul Lewing, Seattle
GURUSHAKTI@AOL.COM on wed 12 jul 00
When you take a glaze formula from empirical to working recipe, the first set
of figures won't add up to 100%. That is the batch recipe. That
weight/percentage will reflect the amount and weight of the ingredients that
you choose to use to satisfy the molecular formula. Once you arrive at the
batch weight, is usually brought to 100% so you can see the relationship of
each material to the other a bit easier. You do that by dividing each
ingredient by the total and then multiplying by 100. (I hope this is right.
I've been relying on Insight's computer calculating for years!). That then
will give you the 100% formula. Both of the resultant formulas are in
percentage and/or grams.
I was taught that the batch weight could be in spoonfuls, cups, etc. but I am
not sure that is correct. I'm sure some of our resident glaze gurus will
clarify this a bit more. Even if true, I never could figure out how to
measure something like 1.17 parts in a teaspoon! :-(
Paul Taylor on sun 16 jul 00
What you say is true. There are some very complicated books out there.
We all tend to design our own methodology for glaze testing. We think
as imperialists our way suits all, so lets invade Vietnam or some heat
stricken African country. We write the memoirs and sing 'I did it my way'.
Same with glaze calculation and methods. As many methods as there are
potters - all sworn by.
So what are you going to do.
Most glaze recipes are put in percentage form by dry weight for either
all or the main ingredients.
The opacity (titanium tin zircon etc) , coloring oxides (stains iron
cobalt etc) and any other little addons like bentonite are sometimes tagged
on at the end as a percentage of the rest of the glaze.
As glaze recipes get passed along they tend to loose the indication that
the additives are extra so the recipe no longer adds up to a hundred,
loosing the percentage quality. Of course you could for forms sake make the
recipe add up to a hundred, but that would be an unnecessary bother and the
numbers you come up with might not look too whole any more. A lot of
recipes do not add up to a hundred just near enough.
Some pedantic types do give their recipes in exact percentage parts which
are inconvenient when weighed out in grams. Some recipes are even
crazier- when the recipe publisher has the recipe in imperial weights and
converts it to metric and then to a percentage. This is why some recipes are
published as a batch weight ( The amounts you bung in the big bucket). The
recipe is often in pounds and ounces but the numbers are tidy and you are
expected to do your own conventions to test it. The proportions are what
count but it is a little off putting to get a recipe from a friend that does
not add up to the hundred or is in batch weight form.
When making up a test recipe in 100 gram form I round all my numbers to
the nearest half gram. This is ok and I expect a little fussy, except for
the coloring oxides and those little additives where it is better to get the
amounts as accurate as possible. Most disappointment come with this, because
these, as with all ceramic materials, are never the same from place to place
so you have to be prepared to make changes. There are countless forms of red
iron oxide etc but the recipes seldom specify which one so a few line
blends may be necessary . You get better with experience at spotting the
essential materials and some glazes are more fussy than others.
Gerstly borate is a prime example of difficulty. The the mineral looses
some of its weight when fired. The substitutes do not contain these
materials like chemical water that burn off in the firing, so you put a
little less in. Of course if you Know your glaze chemistry you will know
exactly how much. And if you have a few years experience you will know
roughly how much to add or take out for luck because the manufactured
material does not melt quite melt the same due to differing particle size.
Another thing to trip you up is that few recipes come with their cooking
instructions. A rule of thumb is that electric firings on the whole are
pretty standard about eight to ten hours. They cool down at the same rate
and ramp rates are usually included for the special effects, but old
reduction firings can vary considerably. A copper red glaze with out the
firing cycle is use less unless you get lucky. I use the Derrick Emmses
recipe it was designed in Stoke Collage. When I discovered this I used a
quicker firing and cool than you would normally expect for a red and it is
beautiful, but woe betide you if you fire it wrong and the recipe leaves
very little leeway for mistakes.
I don't want to be there when the police burst into your workshop while
you are weighing white powder on one of their confiscated machines. I use an
old science beam scale which is so accurate that I have to govern it down.
Any school science balance scale should do - if they still use them. Get a
scale you feel confident of the amounts up to half gram even. If you have to
spend money. If you want smaller amounts weigh out a gram and split it
visually like they do in the films with drugs on a mirror.
If you are doing line blends it is sometimes convenient to do them by
volume- after you have added the same amount of water to each glaze. Line
blending enthusiasts will speak in volumes but essentially they are still
doing things by weight.
There are two schools of thought for glaze chemistry one is Seger
formulas or ultimate analyses , the other is called proportional analyses.
There were some comparative discussions about the two when Nigel Woods
introduced the method in his book Oriental glazes. I prefer the proportional
analyses method because the trace elements that are most important in the
glazes I like are usually only published in proportional analyses form, but
they could be converted to the other system. Also I am not allergic to
simultaneous equations. The Seger formulas are the ones for people who
prefer arithmetic to percentages. I can struggle through Seger's method
Michael Caredew's pioneer pottery explains the calculations very well and
you can get a glaze program from Tony Hanson among others.
Many things in glaze calculation need only be known about so you can
access the complications if you need them. I would not take the books too
religiously. Many people give you a sound methodology, but if you ask them
how they learnt you find out that they started with a few recipes and learnt
the facts as they applied to their predicament. the 'foolproof patent
methods' work but I think just bumbling along is a very efficient way to
learn, if you are willing to enjoy the scenery of a round about trip.
Have you noticed that many so called efficiencies are uncomfortable
to do in spite of their faultless logic . I will send you some more info in
an attachment - it has a lot to do with getting the recipe right .
looking through this letter I am doing what I think most definitive
works on glaze do - scare the life out of the reader. The only rule is do
not poison any body. If you use lead at all get the glaze tested. In this
country that is the only legal requirement. Some potters are concerned that
other substances are also harmful. These people have higher moral standards
in this way than I. Fortunately this is a matter of choice until science
shows definitively otherwise.
If you bung 85 parts of cornish stone or feldspar with 15 parts
whiting fire it from cone ten to cone severn you will get a glaze. If you
throw other interesting stuff in a test and fire it all up on a pot with a
tile underneath you will get an interesting glaze sooner or later. You are
not obliged to make detailed notes but it helps if you do - just in case you
stumble on something interesting. Glazes calculation is only a help to this
spirit - have fun.
> From: Lizacat29@AOL.COM
> Reply-To: Ceramic Arts Discussion List
> Date: Tue, 11 Jul 2000 11:32:46 EDT
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Beginner (very) Glaze ?'s
> Ok..I've been raku firing with my local pottery supply's already mixed glazes
> (a copper sand, white crackle, yukio's rainbow, & lithium carb) for about a
> year. Feeling restive, I want to strike out on my own and mix glazes, I just
> don't quite know how to start. The books just don't seem to get as
> elementary as I need to be at this point. It's the numbers..they have me
> completely mystified!
> I look at recipes and I see percentages (obviously the ones that add up to
> 100) and then "batch". Those numbers are the ones I don't understand. Some
> add up to 100 just like the percentages, and others add up to odd numbers
> like 8.63 or 120. How does that work? And sometimes, I have no idea what
> the numbers mean. For example, some books say at the beginning of a large
> list of recipes: " some are by volume and some are by weight" but I can't
> always tell which is which.
> I understand volume is like teaspoons, measuring cups etc. and weight is
> grams etc. But, which does "batch" refer to? And do I have to invest in a
> gram scale to make my own glazes or can I just measure? I assume that for
> consistency, one must be quite accurate, so is weight better? Gram scales
> seem expensive, what is the best kind to buy, can you get them cheap? I also
> have read about conversion programs to convert recipes from one kind of
> number to the other but I first need to know what all the numbers mean in
> terms of what I do with them once I have them, right?
> How big a batch would I want to make up to start?
> Another question..Gerstley borate seems to have disappeared off the face of
> the earth..Why is that? Is it a natural mineral that is all gone? And I
> read the substitution articles with interest, but so far, no one seems to
> have a great deal of success. So, would you recommend, since I'm just
> beginning, not even trying any recipes with gerstley borate (assuming I could
> even purchase it) because eventually it won't be around?
> Thanks in advance for your help!
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