James L Bowen on fri 25 aug 00
When mixing the 1000g test batches I found it useful to use a 500ml =
graduate to measure the water for adding the dry chemicals. This usually =
made a thick slop that I could then add measured amounts of water to get =
to a sievable glaze mixture. After a few samples I could get a feel for =
how they would respond to small additions water.
My question is how can we determine when a glaze needs more water than =
the next based on the recipe? For example, Malcom's Shino seemed to use =
a lot of water to make it fluid. I assume it is because of the high =
percentage of soda ash (17%). The ash glazes seemed to need more water. =
Same for the Raku glazes containing high amounts of Gerstley Borate.
How can we determine how thick the glaze slop should be based on the =
ingredients in the recipe and the description of the appearance of fired =
pots?
We see descriptions such as single cream, double cream, thin milk. I =
don't use cream. Most glaze recipes don't have the specific gravity =
supplied, so a hydrometer wouldn't be of much use without some general =
concepts based on knowledge of how certain ingredients behave when mixed =
with water.
Wade Blocker on sat 26 aug 00
James,
I have found that transparent, or gloss glazes need to be thinner ,
i.e. more water added, than matt or dry glazes which need to be thicker
therefore less water. I generally do not immediately add all the water
required when mixing a glaze, but save some,to rinse the sieve and
container in which the glaze was mixed. Then I add this "rinse" to the
glaze mix. Mia in hot ABQ
John Rodgers on sat 26 aug 00
In James Bowen's post, he mentioned glazes of creamlike and
thin-milklike consistences.
I will address matters affecting consistencies of glazes.
When you are talking double cream, cream, or thin milk, you are actually
talking measures of density, specific gravity, and viscosity.
If you want to consistently repeat the performance of a glaze of double
cream viscosity, you need to know something about the physical makeup of
the glaze. That is, how much water is present, how much solid material,
etc. Since glaze is composed of solids suspended in a liquid, some
measurements need to be taken.
Make yourself up a batch of glaze that is nearly dry and slowly add
sufficient water to get it to the consistency you consider to be, for
example, cream-like. You need to have enough to fill a pint jar or a
1000 ML flask. I prefer using the flask as it is based on 10 and easy to
work with. But if you have a pint jar the principle is the same.
Keep in mind that Water is the standard against which specific gravity
is measured, and it is taken as the number "1".
Proceed as follows:
1) Weigh the flask or the pint jar.
2) Write the weight down.
3) Next fill flask or jar with water. Weigh the water and jar or water
and flask together.
4) Write it down.
5) Subtract and find the difference. In a 1000ML flask the water will
weigh right at 1000gms. Easy---based on 10. The pint will be about a
pound......"A pint is a pound the world around!" Close enough for this
work.
6) Fill the flask or jar with glaze and weigh.
7) Subtract and find actual weight of glaze less the weight of flask or
jar.
8) Divide the weight of the water into the weight of the glaze. The
number you get is known as the Specific Gravity and will probably be
greater than 1. Like maybe 1.5.
It is expressed like this. "The Specific Gravity of water is "1". Your
glaze number may be 1.5. Then you could say "The Specific Gravity of
this glaze is "1.5". All of which simply means that your glaze weighs
1.5 times the weight of water. This can give you a clue as to the
relation of the water to the solids. And possibly the "Viscosity" or
pourability of your glaze.
Your glaze of creamlike consistency is going to have a certain specific
gravity (SpG). If you increase the amount of water, the SpG will go
down. That is, your 1.5SpG glaze will become maybe a 1.2SpG glaze. That
would be more near the thin milk consistency. Reduce the water content
or add more solids, the SpG increases and the glaze will become more
like double cream or even cold mollasses.
If you have mixed your glaze at exactly the pourable consistency you
want it, calculate the SpG and record it for posterity. It is your
reference data for future work. You can work out the SpG for your own
version of double-cream, cream, and light milk, then in the future when
you mix a glaze you can get it right on every time.
If you have a bucket of glaze and it sits a while, the glaze will
settle. Also, over time water will be absorbed and the relation of
solids to the liquid will change. Usually the liquid will be used up or
evaporate. So how to adjust back to that perfect cream-like consistency
for a particular glaze? Well, I hope you kept notes.
Don't stir the water back into the glaze, but pour most of it off. Stir
the glaze. Then measure the specific gravity. It should be much greater
than the SpG of your perfect glaze . So add a small amount of water,
check the SpG again. Keep this up until you have it right on the target
SpG. Now you are ready to go.
Now a glaze can have the exact perfect SpG but pour like glue. So the
next step with a glaze is to measure the pourability or viscosity. That
is the rate at which the glaze flows.
The easiest way I know to measure viscosity is to buy and use one of the
little plastic viscosimeter funnels from Laguna Clay, or Axner or other
supply house. Costs only a couple of dollars. You also need a clock with
a sweep second hand.
Get your specific gravity exactly right on.
Place your finger over the calibrated orifice at the bottom of the
funnel.
Fill the viscosimeter funnel to the top with glaze.
Note the time of the sweep second hand
Release your finger
Allow to drain to only two or three drips fall.
Note the time.
The measure of the time it takes to completely drain is a measure of
viscosity or flow rate of a mixture of known SpG.
So now say you have a Glaze of SpG of 1.5 with a Viscosity of 45
seconds.
Is this good or bad??? Only you know. You have the experience with the
glaze. You now have a means of measuring every glaze before you apply
it. You have quantified some of the glaze parameters and by doing so
have gained some measure of control. If you have a really good glaze
firing and get out a really good pot, you now can repeat the numbers of
the glaze and should be able to get similar results every time.
The last item that needs looking at is how to adjust a glaze viscosity
without adding water.
If you have a glaze whose SpG is right on, but whose viscosity is off,
adding water will simply reduce the SpG. That won't work. So what to do?
Well, say the viscosity flow rate is 1 minute and your perfect flow
rate for this glaze is 45 seconds. The SpG is right on, so you know you
can't add water. Well add a few drops of Sodium Silicate to the glaze.
It will loosen up and flow well. ***A CAUTION!!!!"*** The Sodium
Silicate can cause your glaze to settle out instantly into a solid
rocklike hardpan on the bottom of your bucket and you will be really
hard pressed to stir it up, and even if you can, it won't stay
suspended. So there is a tiny window you will be working with.
Another thing. If the glaze is close to right but seems a tiny bit runny
on application, a pinch of epsom salts sometimes will tighten the glaze
right up slowing the flow rate
Finally, The principles outlined here also apply to working with
slipcasting clays as well. You have to consider that many glazes contain
some clay ingredients, and that is what the chemicals mentioned above
work on.
I will close with saying that there are ways to take a given glaze
already mixed with water, and actually calculate the amount of solids in
it, without having to dry the glaze completely out. This allows you to
work up a dry batch of ingredients to add to an existing glaze that
perhaps has been over zealously watered down. That discussion goes well
beyond the scope of what I wanted to present here.
Hope this discussion is of benefit to someone out there.
John Rodgers
Birmingham, AL
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