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questions about glazes (lofire)

updated sat 3 mar 01


Sharon Miranda on wed 28 feb 01

Dear clayarters: bear with me, I'm trying to learn as fast as I can, but
there are not enough hours in the day. I hope someone can give me advice.
Could someone tell me
A) why the following glaze tests dissolve into very little water and become
very difficult to apply and get a good coating on the bisque (they seem to
slide off and repeated dips get worse) and
B) how do I improve their application? It seems that they are either too
thick or too thin.
C) some of them settle out in the bottom of the container. Would adding
1-2%bentonite help this?

(All glazes are from Val Cushing's book)
All glazes were applied to bisqued terracotta and glaze fired to 05. Aside
from being too thin, they all seemed to work as described. I made 100 grams
of each (too little, I guess, to really get an idea).

VC- opaque satin
frit 3124 72
Neph sy 12
flint 10

VC alkaline mat
frit 3110 50
ball clay 10
flint 10
whiting 10
barium carb 15
zinc ox 5
copper carb 2

V C 5000 stony mat
frit 3124 77
kona 4 spar 14
whiting 7

Additional questions: would a hydrometer help determine exact thickness of
the glaze?

To some of these tests I added 4% copper carb and the surface changed from
satin or semi matt
to more glossy. What do I do to get the satin surface I want AND the color?

Thank you! I've read with such interest the great contributions recently on
methods of glaze testing.
Sharon Miranda

Snail Scott on thu 1 mar 01

At 04:18 PM 3/1/01 -0700, you wrote:
> When you mix glazes try to use 100 grams of water to 100 of glaze.

High-frit glazes like the ones Sharon posted generally
take less water. High-Gerstley glazes, on the other
hand, are jello at that ratio, and require more. The
'equal' ratio is a fair rule of thumb, but each glaze
will be different.

Martin Howard on thu 1 mar 01

I will try to answer this part, as the glazes mentioned use US RMs that I do
not use
the glaze?>
Yes. You can buy or make your own.
You can use Relative Density/Specific Gravity or Baume readings. They are
translated in the following list
Baume SG Baume SG
38 1.36 0.00 1.00
39 1.37 2 1.01
40 1.38 4 1.03
41 1.39 6 1.04
42 1.41 8 1.06
43 1.42 10 1.07
44 1.44 12 1.09
45 1.45 14 1.11
46 1.46 16 1.12
47 1.48 18 1.14
48 1.49 20 1.16
49 1.51 22 1.18
50 1.53 24 1.20
51 1.54 26 1.22
52 1.56 28 1.24
53 1.58 30 1.26
54 1.59 32 1.28
55 1.61 34 1.31
56 1.63 36 1.33
57 1.65
58 1.67
59 1.69
60 1.71
61 1.73
62 1.75
The first two columns give the spread of values available in most bought
Baume version hydrometers. The second two columns give the values between 0
i.e. water and the start of values more needed for glazes.

After breaking three bought glass hydrometers I have made my own out of
plastic specimen phials. The top screws on tight, so no leakage. The bottom
is cut away so there is no cavity underneath the phial to hinder the
Inside I put a short cut from a thick screw, almost the diameter of the
phial. Then add a little wax to keep it in place. Test to get the right
amount of metal screw in the bottom, so that the hydrometer just stays
50 degrees Baume, which is what I need for most glazes, lies about 10mm
below the bottom of the screw top. Mark your favourite required levels as
you want.

Some glazes are thixotropic and do not give a proper reading. You can tell
them by how they stick to the surface of the hydrometer and give stupid
readings. Those glazes then need to be weighed. Weight of volume of glaze
divided by the weight of the same volume of water gives the Relative Density
or Specific Gravity.
So, weigh the 1 litre jug. Wj
Then weigh it with 1 litre of glaze (after well and truly stirring it).
W(g+j)-Wj = Wg
Wg/1000=RD(or SG) of glaze.

Hope that is helpful.

Martin Howard
Webb's Cottage Pottery
Woolpits Road, Great Saling

Cindy Strnad on thu 1 mar 01

Hi, Sharon.

First, you're using too much water. Let the glazes sit for a day, then
suction or ladle off the extra water on the surface. Generally, having
glazes the thickness of heavy cream works for me, but glazes vary. Some need
to go on thin, others, thicker than is usually advisable.

Second, yes, one or two percent bentonite would help these glazes, which are
rather lower on clay content than I generally work with. If the glaze
contains no clay at all, it's my understanding that bentonite will be

It's best to mix up the bentonite beforehand with the other dry ingredients,
or to mix it into a bit of water before adding it to your other stuff. I
confess that, for small test batches, I just sprinkle it in a little at a
time and blend with an immersion blender, but don't tell anyone.

Hydrometers are of limited effectiveness in determining optimum glaze
thickness. I have one, and haven't found it to be much help. Good to have
for terra sig, though. After a while, you get a feel for how thick your
glazes need to be. In any case, you'd need enough glaze to float the
hydrometer, and 100 grams isn't going to do it unless you want to pour it
into a graduated cylinder.

Adding oxides often causes glazes to go from matte to satin or shiny. This
is due to the increased melting of the glaze caused by these oxides. It
means that your matte surface is the result of immaturity (underfiring).
That's all right if you don't plan to use the glaze for functional ware. If
you want a matte you can use on the dinner plates, though, you'll want to
look at crystalline matte glazes. These require slow cooling and give waxy
rather than stony surfaces.

If you want a dark green, try copper oxide instead of copper carb. Or you
might try chromium.

I know you've heard it all before, but in case . . . . you *must* wear a
well-fitting, high-quality mask when mixing these glaze chemicals. Be aware
of the dust which will accumulate on your person. Vent your kiln. :)

Hope this helps,

Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730

Wade Blocker on thu 1 mar 01

When you mix glazes try to use 100 grams of water to 100 of glaze.Have
a small plastic container and see how high the water level is for 100
grams. Draw a line with indelible ink at the water level. Then you will
know what quantity of water to use next time, without having to weigh it.
If the glaze is supposed to be a matt glaze use a little less water. I use
1% of mecaloid in my glazes.You could also use a spoonful of CMC solution.
It makes the glaze adhere better to the ware. I only bisque my ware to
cone 06. The higher the bisque is fired the less it will absorb glaze. Do
you wet your bisque ware before glazing? That again would be detrimental to
glaze adhering.If your glazes contain too much water, just let them sit
without cover for a day or so, allowing the water to evaporate. Hope this
helps. Mia in ABQ

Jonathan Kaplan on fri 2 mar 01

on 3/1/01 10:25 AM, Martin Howard at martin@WEBBSCOTTAGE.CO.UK wrote:

> I will try to answer this part, as the glazes mentioned use US RMs that I do
> not use
> > the glaze?>
> Yes. You can buy or make your own.

While even with the correct SG of a glaze, thickness is also determined by
the length of time the pot is subjected to the glaze by dipping, pouring, or
spraying as well as the absorbency of the pot also. Some companies do
market a small dial gage for determining the thickness of the glaze on the
piece. Nonetheless, glaze poise is determined by a combination of things,
including the specific gravity of the glaze, the viscosity of the glaze, the
absorbency of the clay body, method and timing of application, operator
error, and others in cluding what additives are included in the glaze slop.

Martins chart is quite useful for those using a hydrometer to measure the SG
of a glaze or a casting slip. Hydrometers are often inaccurate and less than
reliable in providing consistent SG numbers.

While I am sure that few potters on this list have access to or own a
Brookfield Viscosimeter, short of this the best and most accurate method of
ascertaining correct SG is to weigh the glaze or slip and do some very
simple math comparing it to the same volume of water and arrive at the SG as

1. Weigh a container empty.
2. Stir and mix the glaze wel. Fill the same container with glaze,
compensating as best you can for the miniscus.
3. Subtract the weight of the container from the weight of the glaze and the
container yielding the weight of the glaze
4. Clean the container and fill it with water, again, compensating as best
you can for the miniscus.
5. Subtract the weight of the container from the weight of the container
with the water yielding the weight of that same volume of water.
6. Divide the weight of the glaze by the weight of the water and you will
have the correct SG.

Remember that glazes are suspensions, or better, colloidal suspensions and
hydrometers do not work well, if at all, in such a situation for determining
SG. Hydrometers do work in solutions.

The best way is to weigh the glaze or slip and follow the steps above.



Jonathan Kaplan
Ceramic Design Group
PO Box 775112
Steamboat Springs CO 80477

Plant Location (use for all UPS, Common Carrier, and Courier deliveries)
1280 13th Street
Steamboat Springs CO 80487

Lili Krakowski on fri 2 mar 01

Ok. But be advised: ALL of this info is found in BOOKS such as Harry
Fraser's Cearamic Glazes for the Potter. Hint: call The Potter's Shop
order that book as well as the HF's book on ceramic faults and their

1. Following HF I add 3% Bentonite and about 2% Epsom salts, or some
calcium chloride solution to ALL my glazes.

2. Glazes should be applied about the thickness of a nickel. From a dime
to a quarter is the range. (Don't come at me with exceptions! Please!)

3. Consistency of raw glaze in bucket should be about that of light
cream. For Brits: the equivalent of cream atop the milk bottle. Not
double Devonsire!!!) For dieters: the thickness of bottled French

Lili Krakowski