Tony Hermsen on mon 27 may 96
Re: white spotting due to pinholing
I know that this topic has been much discussed in the past but it seems to
remain a nemesis for many potters working in the majolica technique. I would
like to re-open discussion on this topic and invite other potters with majolica
expertise to add their valuable insights and solutions.
Perhaps, I should begin by explaining my own approach. I currently use a
commercial clay body, the Red Art Terracotta MD body from Clay Art Center,
Tacoma. I find this body to be very forgiving of my particular handbuilding
techniques as well as becoming very strong and dense when fired. I biscuit to a
small cone 06 and glaze fire to cone 05. Most of the white spotting problem
occurs on tall or vertical pieces. Flat or horizontal pieces with a generous
foot or area of bare clay usually have no spotting, only when pieces gain
vertical height with a proportionately small area of bare clay does spotting
ocur to the point of becoming unacceptable. The culprit here is gas released
from the clay body and passing through the molten glaze. This gas pushes the
overglaze color aside and when the pinhole heals itself a white spot of glaze is
left behind.
Why do pieces with generous unglazed areas not develop white spotting? Gas
bubbles will tend to take the path of least resistance through the relatively
porous clay body rather than the sticky molten glaze. Where does this gas come
from? Earthenware bodies are notorious for their carbon and sulphur content.
During firing, these substances can release a considerable quantity of gas. Most
of the carbon will burn out by 900 C. so this should not be a problem after
having done a 1016 C. biscuit firing. Sulphur, however, does not completely burn
out untill a temperature of 1150 C. is reached. This is above the 1041 C. glaze
firing that I use and is well above the usefull biscuit temperature for most red
earthenware bodies.
So, how does one minimize white spotting given these conditions?
-make forms that leave plenty of unglazed surface area. Personally I find that
this can be a restricting way to work.
-slow glaze firing. Burn the gases off slowly enough so that the force of the
escaping gas is minimized thereby preventing large bubbles pushing through the
overglaze color. I find this to be most effective since it allows gas to escape
through the porous glaze (before it begins to sinter) and also keeps to a
minimum the pressure of the escaping gas so that it disperses without forming a
large pinhole during the glaze melt( the bigger the pinhole the bigger the white
spot in the overglaze decoration ). This is especially important from the time
that the kiln starts to show color until the final 1041 C. shut off. But the
question remains - how slow is slow? Currently I use a 16 hour glaze firing
cycle with at least 4 hours of slow temperature rise from first color to shut
off. Maybe this needs to be longer still?
- a wash of alumina applied between body and glaze. Is this practical? Has
anyone tried this? How would one maintain proper glaze fit? If it works, why
does it work?
-use a kiln vent to draw air through the kiln to constantly clean the
atmosphere in the kiln. This seems to make sense but I don't have one of these
gadgets. Does anyone have practical experience with this?
-change the clay body. OK! What redware claybody does not contain carbon or
sulphur? Maybe I am wrong here but it is my understanding that they all do. Any
suggestions? I have heared that substituting as much alumina as possible for
flint in the body markedly reduces outgassing. Again, I don't know why this
would be so or if it is even possible to do with an earthenware body. It sounds
like a stoneware potters kind of thing. Let's hear from you clay body experts on
this.
Well, I have been long winded enough . Now it is your turn. Looking forward to
your thoughts and info.
Conqueror of the white spot,
Tony
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