Jonathan Kaplan on mon 22 jul 96
OK Glaze Gurus!!
I use a cone 3-5, even cone 6 white versatile majolica (electric firings)
for a major client and this glaze shows off its drips quite well. In fact,
it is contributing to some unwanted seconds.
The formula:
EPK 9.61
Whiting 15.86
Flint 20.7
Custer 35.81
Zinc Ox 9.01
Zircopax 9.01
The glaze is a great white, but the application is crutial and I am
wondering about the glaze poise, or the set of the glaze on the ware. I
think the glaze is adequately flocculated with a solution of bentonite and
Flocs, and runs at an SG of about 1.6. We have experimented with various
dipping strategies, to no avail.There are still streak marks, thick and
thin marks, from dipping. The pots are mugs, all the same, and all glazed
the same. Usign tongs with sharpened points, the mugs are dipped in an arc
motion to glaze both the inside and the outside and as they are raised from
the glaze bucket, on an angle, the interior glaze slowly decants out. There
are still streaks and uneven parts on the ourside surface, but only on the
side that the glaze runs off. The opposite side is perfect.
Any suggestions? TIA
Jonathan
Jonathan Kaplan
Ceramic Design GroupLtd./Production Services
PO Box 775112
Steamboat Springs CO 80477
(970) 879-9139 voice and fax
call before faxing
"Show up, pay attention, tell the truth, don't be attatched to the results!"
peter pinnell on mon 22 jul 96
Hi Jonathon_
It sounds like the problem is one of glaze formulation rather than of
application. I would guess that the glaze has too high a surface tension
because of the (relatively) high amount of zinc. Looking at it chemically
it looks like a higher fire glaze that has been brought down in
temperature by the addition of zinc, so the zinc can't just be left out.
I guess the easiest solution is to substitute an oxide with a lower
surface tension for the zinc. That would point to lead, barium and
strontium. I assume that lead and barium are out of the question, so I
did a quick substitution using strontium carbonate.
Revised cone 3-5 Majolica
Custerspar 36, whiting 16, strontium carbonate 17, kaolin 10, flint 21
plus zircopax 10.
The additions of bentonite and floccs would definitly help this glaze for
dipping.
Good luck!
Pete
Ken L Russell on tue 23 jul 96
wondering about the glaze poise, or the set of the glaze on the ware.>
Jonathan, sorry I don't know beans about glaze but with this white at c6,
does it allow engobes to "blend through" the white so that colors such as
iron oxide browns or black copper oxide greens are pronounced? Also, at
1.6, how "runny" is it? Also, have you tried combinations of tin and
zircopax? More zirc seems to make my base white glaze over iron oxide a
grayish-dirty brown while tin tends make a more burgundy brown. Sorry
for all the questions but I'm still on a destructive quest for a
no-brainer glossy white base glaze that comes out perfect everytime
.
Ken Russell
The Arlington Pottery
gone2pot@juno.com
Don Kopyscinski on wed 24 jul 96
Hi Jonathan,
As Pete pointed out, the fact that this glaze is a stiff one causes the drips to
show more readily. Softening the glaze in the fire is one way to go. I would
like to see you do some application tests first to see if that is needed. You
say you've used bentonite and flocs (an effective combination, as well as an
*extremely sensitive* one) to flocculate the glaze and you have a S.G. of 1.6.
Here is the scenario I see (look closely at the glaze as you're glazing and tell
me if this is the case):
1- You dip the mug in the glaze. (a rather dense flocculated one)
2- On removing the mug, you will see that the glaze may be hanging onto the wax
for a second or two, then sheeting off.
3- This sheet then cascades down the side of the mug, depositing that second
troublesome layer
The problem as I see it, is the high poise of the raw glaze (you knew that, as
you mentioned it and you hit it dead on).
So.....what you need is more fluidity and lower adhesion. Striking a balance
between S. G. and the level of flocculation is the key to solving many
application related defects. I would also assume that if you were to put a dry
finger into that well mixed glaze and remove it, your finger would be well
coated and not a drop would come off of it.
You could use less flocs, or to an extent more water, and a longer dip time.
What this would accomplish would be:
1- less pick-up of the glaze per second
2- an immediate release of the glaze from the waxed portion
3- a faster flow of that glaze sheet draining off the pot
4- lower adhesion of the glaze so when it does flow it would deposit a much
thinner layer where it is draining
5- a greater saturation of the bisque when dipped, reducing it's porosity and
propensity to take on more glaze as it drains
You may be able to just add enough water to the existing glaze until you get the
application properties you need. What you're looking for is a near instantaneous
release of that glaze from the wax, a cascading of the glaze layer in a sheet
but one that travels rather quickly down the pot, draining well. Flocculation
tends to give nice application qualities to a glaze and it's striking the right
balance that will give you the optimum qualities. A slightly flocculated glaze
gives a nice even layer, but allows the glaze to smooth out and the drips are
less defined. The drying period is also extended a bit. A shake of the tongs at
the right moment will let the edges feather out, softening the edges of drips
and often smoothing them out quite nicely. Too much fluidity, will give you a
series of drips for other reasons, as the thin glaze breaks into many individual
streams of glaze as it drains off the pot. A very thin and fluid glaze also
tends to dry rather quickly, not allowing time for the edges of the drips to
soften. If you've ever had a highly *defloccuated* glaze caused by soluability
of lithium carb. or have added too much deflocculant to a glaze, you know how
the smoothness of application deteriorates, often with the glaze drying almost
instantly, freezing those drips in place as they drain from the pot. So..... too
much in either direction and you're asking for trouble.
It's a delicate balance and you need to watch how the glaze is flowing and let
it tell you when you're there. A quick check to let you know you're near the
target is to dip an index finger to the second knuckle and remove from the
glaze, letting the glaze drain naturally from it. No drops off the finger
indicate too stiff a glaze slip, one drop to two seem to be the range I like to
shoot for, with one off the finger and another just hanging but not quite off, a
good place to start. I know that a viscometer is more accurate and that a glass
plate is better than a finger, but I think this is enough to get you in the
right range then you need to let the glaze and it's behavior as it drains from
the pot and those pesky drips be your guide to hit your mark.
The glaze may be over flocculated. If this is the case, drain off the water in
the glaze now after it has decanted for a day undisturbed (I want to get rid of
some of the flocculant so you can start fresh), measure it so you have a guide
as to how much to replace. Add back about 75% of the volume you drew off with
fresh water, then *slowly add more* until a dipped finger will yield one, two,
or three drips off (this is your range and you need to determine which one gives
you the best result, starting in the center and adjusting if needed) without
shaking it off. You may need to add more water than you drew off to get to this
point.
Once you have the fluid properties of the slip where you want them, dip a series
of tiles. Measure the thickness if you can and note it on the bottom of the
tile. If you can't measure the glaze coat, just note the number of seconds
dipped ranging from 1 to 15. You will then find your target range as far as dip
times to yield the thickness of glaze coat you need for optimum glaze character.
I shoot for a 5 to 10 second dip time depending on the glaze and the desired
coat thickness needed (ranging from .005 to .015 inches in thickness for dry
glaze coat). Once you've hit on it measure and record everything, so you can
always get back to where you are now. Set that as your standard.
You will still need to deal with that last drip hanging onto the rim of the mug
by first quickly shaking the tongs just before you release the mug, then:
A- blowing on it at the right time to smooth it out
B- using a wet soft brush to fan it out a bit before it is completely dry
C- or if I know you..... a stream of 5 psi compressed air positioned at 2cm
directed at an oblique angle of 37* to spread that drip over a wide enough area
to thin it out (whatever works as indicated by the result)
My production glazes run a range of 1.39 to 1.59 S.G. The one with the lower
S.G., will actually deposit a thicker coat than the higher one. This is where
the effect of flocculation comes into play. A flocculated glaze's increased
adhesion can greatly effect the dip times and S.G. needed to get the thickness
you desire for your application. Since the drips are the culprit here, use the
fluidity of the raw glaze as your control, then adjust the dip times and if
needed the S.G. to yeild the desired thickness of glaze coat. If you can, work
within the center of your target range, this will give you some lattitude for
the small variations that can occur that you shouldn't need to worry about.
If you find the range so narrow as to be unreasonable in a production situation,
then it's time to soften the glaze.
P.S. And to think....... U.S. Gypsum spent all that time and money hiring
engineers, on research, product development, testing, and quality
control......... to make just freaking plaster......What a waste!!!.....(NOT).
Hope this helps,
Don
Don Kopyscinski
Bear Hills Pottery
Newtown, CT
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