ClayCoyote@aol.com on fri 12 apr 96
There was quite a bit of discussion on this about a year ago. If I remember,
the net takeaway was that a floating hydrometer is highly inaccurate (one
person mentioned assigning measurement as a class project and getting no 2
measurements alike). The problem is the viscosity of the slip/glaze and that
it is a suspension, not a solution. Since we're measuring the amount of
solids in the suspension, the best way is to measure an exact amount and
weigh it. To actually find the specific gravity, you would also measure an
equal amount of water and divide the two. In a given studio, it's not even
necessary to get to SG, just always use the same volume and work to the same
weight after having measured the weight of a well performing batch. We use a
small 8-oz plastic jar for this, filling it so the glaze just fills the edge
around the top, eliminating, we hope, inaccuracies due to the miniscus.
Did I get this right?
Tom Wirt
The Clay Coyote Pottery
Hutchinson, MN
Claycoyote@aol.com
Linda Arbuckle on sat 14 aug 99
> Speaking of the proper dipping density for pieces bisqued to 04, what is
> the proper density for other temps? Where do you find this info? What
> about proper density for brushing glazes? I've heard some of you guys
> discuss this before, but does it come from your personal trial and error
> or is there something like limit formulas somewhere for this?
>
> TIA
> Cheryl Litman
>
Cheryl,
I think measuring specific gravity of a glaze is just a way to
duplicate conditions that have been successful for you in the past, and
perhaps to give other people a ballpark idea of what you mean when you
say "thick" or "thin" glaze.
So many things influence glaze thickness, that dipping application is
not exact, even if you duplicate someone else's glaze thickness for
dipping.
If you use a different clay body, it may be more or less porous fired to
the same bisque temp as someone else's clay body and absorb glaze
differently.
If you bisque to a different temperature with the same body, absorption
may vary.
If you dip or pour longer when glazing a piece, even tho all else is the
same, you'll have a thicker glaze coating. Some pieces are easy to
glaze, others take more time to manipulate.
I dampen my work before glazing to remove dust and make the bisque a bit
less thirsty (fewer air pinholes in raw glaze). Dampening more or less
effects glaze uptake during dipping.
If you have 2 pieces of your own work bisqued to the same temp, same
clay, but one has thick walls, one thin, and both are dipped the same
amt of time, glaze coat may vary. The water in the glaze moves from
greater concentration to lesser (i.e. into the bisque), leaving the
glaze chemical suspended in the water on the surface of your pot. Thin
walls waterlog sooner. Thick walls offer more space for water to be
absorbed and will glaze a bit thicker in the same time.
That said, you just try to pay attention to conditions, duplicate what
works as best you can, and try not to do the things that bombed again.
Some glazes demand thicker application, some thinner. Generally, I think
of "postcard thickness" as middle of the road. Some glazes are less or
more fussy than others about thickness. John Tilton, a potter near
Gainesville, FL, has told me his crystalline glazes (which he does
beautifully!) are very fussy about thickness, and he's tried using a
glaze thickness measuring device to check on his application. (Note: I
think these have been discussed on the list some time ago. Also Axner
sells them.). I just dip my arm in my majolica glaze, and watch how it
runs off, expecting to see the hairs on my arm but not the skin
underneath when it's adjusted to the right thickness for glazing. This
is a bit inexact to convey, so specific gravity might be more helpful to
share w/someone, but would be no guarantee he/she would get the same
application. Art life is messy, eh? Fortunately, there is a bit of room
for slack. Happy glazing.
If you want to see John Tilton's work:
http://www.tiltonpottery.com/
Linda
--
Linda Arbuckle
Graduate Coordinator, Assoc. Prof.
Univ of FL
School of Art and Art History
P.O. Box 115801, Gainesville, FL 32611-5801
(352) 392-0201 x 219
e-mail: arbuck@ufl.edu
kollin baker on sun 31 oct 04
Hey yall
I recently read an easy way to determine specific gravity of a glaze, it was in C.M.
Of course now I cant find it. I believe it was in the last couple of years and was by Mel J. The article showed one how to use a gram scale. Please let me know if you now HOW or where to get this INFORMATION.
Send it to
kollinkollin@yahoo.com
thanks
Kollin Baker
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John Rodgers on sun 31 oct 04
Kollin, I was about to make commentary about how essential knowledge of
you glaze density is in order to control it's application, when I ran
across your request.
First - remember that a pint is a pound the world around. For water,
that is.
Take a pint mayonnaise jar, weight it, write down the weight. Then weigh
out a pint of water - should be a pound. Then weigh a pint of glaze. Be
sure to subtract the weight of the jar from the total weight of the jar
and glaze. Divide the weight of the pint of glaze by the weight of the
pint of water. The result is the density - or in the older term -
specific gravity - of your glaze.
You can use any container actually. The trick is that you always use the
weights of equal volumes water and glaze. And it doesn't matter if you
use ounces, grams, or pounds.
I find it necessary to know exactly what the density of my glazes are
for best application. For example, I work with Floating Blue. One of my
best glazes. I find I have to have exact control over it's application
or the glaze is crap!! So, I work by the numbers.
The glaze is comprised of exact amounts of chemicals, so why not for the
rest of the process? So after the glaze is mixed, I determine the
density. Experience has demonstrated that for my purposes, I need to
apply the glaze to the surface for a specified period of time, and at a
particular density. If I am dipping a pot in the glaze, the pot -
bisqued to cone 06, must be held submerged in the glaze for 15 seconds.
The glaze must have a density of 1.5, or be 1.5 times heavier than plain
water. If the application process follows these numbers every time, then
I get really gorgeous work every time. If not, I get crapola.
Another number I use is 10 ..... as in 10 spray coats of glaze.. I make
some flat ware, like plates. Experience has taught me that spraying
glaze is the only way to go for making my plates. Again, this requires
the glaze - density 1.5 - be applied in 10 thin coats. Less than 10,
or more than 10, then the color is off.
So knowing the numbers for your glaze is what will give you control over
your glaze process, and improve your consistency and production.
Regards,
John Rodgers
Chelsea, AL
kollin baker wrote:
>Hey yall
>I recently read an easy way to determine specific gravity of a glaze, it was in C.M.
>Of course now I cant find it. I believe it was in the last couple of years and was by Mel J. The article showed one how to use a gram scale. Please let me know if you now HOW or where to get this INFORMATION.
>Send it to
>kollinkollin@yahoo.com
>
>thanks
>Kollin Baker
>
>
>---------------------------------
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>Y! Messenger - Communicate in real time. Download now.
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Jim Larkin on mon 1 nov 04
>I recently read an easy way to determine specific gravity of a glaze, it
was in C.M.<
I had an article in Ceramics Mo. in Dec. 2001 on measuring specific
gravity, with a short explanation on zeroing out your gram scale, or
accounting for the weight of the container if you don't have a tare poise
on your scale. I also find it personally helpful to use a small mouth
container. A little error in under/over filling is not as significant
that way. A beer bottle will work fine.=20
John Britt's new glaze book has a good section on specific gravity also.
Jim
=20
Jim Larkin=20
Fox Pass Pottery=20
Hot Springs, Arkansas=20
71901=20
501-623-9906=20
foxpass@aristotle.net=20
www.foxpasspottery.com=20
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