Chris Schafale on mon 9 sep 02
Yes, I use this also. I've been meaning to post about it but was too
lazy to go look it up.
Below is a post from Clayart -- NOT written by me, I hasten to add -
- that explains how to use it. I have edited it a bit for simplicity --
there was more technical info in the original. Use #3 fits the
problem someone asked about.
Chris
Quote:
Brongniart's formula enables one to estimate the amount of dry
material in a given volume of "slop," that is, of a glaze or slip
already mixed with water.
The only measurement that you need to make is to
determine the density of your slop, by weighing one liter of it, in
grams.
Then plug that density value into Brongniart's
formula:
grams of dry matter per liter of slop=(density of slop - density of
water) (density of drymatter)/ (density of dry matter - density of
water)
Because the density of dry matter in most non-lead glazes is about
2500 g/l and the density of water is about 1000 g/l, the formula
simplifies to:
g dry matter per liter of slop = (density of slop - 1000) 5/3
So, for example, suppose that a liter of your glaze slop weighs
1500 g. Then the amount of dry material in it is (1500-1000)5/3 =
833.3 g dry matter per liter. And since the total slop mass is 1500
g, we also know that the glaze slop contains 1500-833.3 = 666.7 g
water per liter of slop.
Here are three uses I have found for Brongniart's formula. I would
be interested to hear of others.
1. Suppose you have a bucket of glaze slop--maybe not even a full
recipe's worth and perhaps a bit thicker now as a result of
evaporation. You would like to try adding, say, 1% cobalt
carbonate
to a sample of it, to tint the glaze blue. But that's 1% of the dry
glaze, and you don't know how much dry matter there is in a given
volume of slop. Mix your glaze very well, including scrapings from
the sides of the bucket, weigh a liter of it, apply Brongniart's
formula, then add cobalt carb at 1% of the dry matter. Simple.
2. Most stoneware glazes that are applied by dipping work best at
a density of about 1500 g/l (i.e. a specific gravity of 1.5) Suppose
you are making up 14 kg of a glaze and want to know how much
water would bring the slop to that density. The amount of water
that would make a liter of slop with a density of 1500 g/l can be
found by using Brongniart's formula, and in fact we have also done
so, in the illustration above, because we assumed for purposes of
illustration that slop density was 1500. The result was a mixture
that for every 833.3 g dry matter contains 666.7 g (666.7 ml) water.
That is, the water added per 833.3 g dry glaze material is
666.7/833.3 = 80%. So, our hypothetical batch of 14 kg dry matter
would require an addition of 80% of 14 kg water, i.e. 11.2 kg ( 11.2
liters) of water.
3. I recently made up a new glaze with the amount of water
calculated to bring the density to 1500 g/l, but what I got was
something too thick to be a normal glaze, and sure enough its
density was too high, well above 1500. Assuming that I had
weighed out one ingredient twice (the ingredients were weighed out
over a two-day period), I calculated the total amount of dry material
in my bucket. It was very close to 600 g above what it should have
been; only one of the ingredients in my glaze was called for in that
amount.
--contributed by Stuart Altmann
Date sent: Sun, 8 Sep 2002 13:31:55 -0700
Send reply to: Ceramic Arts Discussion List
From: Ned Ludd
Subject: Re: mystery glaze
To: CLAYART@LSV.CERAMICS.ORG
> Hi Lily
>
> Instead of taking all the time and trouble to render a wet glaze back
> to bone dry in order to determine its dry weight, why not use
> Brongniart's Formula?
>
> What it does is determine the dry weight of the ingredients in a wet
> slop: slip or glaze. To work the formula all you need is the relatve
> density of the glaze, and an accurate volume of same. Take out your
> electronic calculator and off you go.
>
> The Hamer Dictionary has an comprehensive entry on Brongniart's
> Formula: my copy is away at the studio, or I'd cite it here.
>
> I'd be interested to learn if any other Clayarter uses this method.
>
> best
>
> Ned
>
>
>
> Lily Krakowski wrote
>
>
> >My suggestion--that WHILE LETTING THE GLAZE IN THE BUCKET DRY OUT ENTIRELY
> >you make some tests. Straight line blends. Leave out all whiting in one
> >test, all spar in another, all frit in a third.
> >
> >When the tests are done and the clay-in-bucket bone dry weigh the dried
> >glaze so you can add the right amount of whatever. Good luck
> >
> >
> >Steve Burtt writes:
> >
> >> Wonder if I could get a little help with a glaze problem? I made a
> >>mistake when mixing a bucket of butterscotch and the result was
> >>really nice, but I do not know what I did to it.
> >>
> >> Here's the original glaze: Butterscotch:
> >> Custer Feldspar 28
> >> Frit 3134 32
> >> EPK 14
> >> Silica 19
> >> Whiting 7
> >> Rutile 4
> >>
> >> This glaze makes a glossy, translucent butterscotch that gets
> >>creamy and white when thick. The strange bucket that I made up came
> >>out opaque, semi-glossy, and thick; did not go white when thick.
> >> My original thought was that I used G200 instead of Custer,
> >>but I did a test and G200 does not make a noticable difference.
> >> I am perfectly willing to do some testing if someone can tell
> >>me what I might have added too much or too little of.
> >> Thanks.
> >>
> >> Steve Burtt
> > > Ocean Springs, Mississippi
>
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Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh)
candle@intrex.net
http://www.lightonecandle.com
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