It's The Pits! In The Glaze

[graybeard]

Hi All,

 

I have yet another question. I have some pieces that I bisque fired to a ^03,

then coated with 3 coats of a ^05 amaco glaze and fired to a ^05 Per package

instructions. (actually it was a little over fired, the ^05 witness cone was

about 1/2 inch from touching the shelf)

There were 2 or 3 small pits in the finished glaze where the glaze didn't flow

so I recoated the spots and re-fired to a Good solid ^04. I still ended up with 2 very small pits smaller, but still there. I called a friend who is a

professional potter and really knows the craft, (I hope he doesn't mind my

calling him a friend) He said I should re-cover the spots and re-fire the piece,I am of course going to do that.

 

My question for the community (you guys) is: is there ANY way to set up a MANUALkiln to do a temperature hold, or soak or whatever you want to call it

To give the glaze more time to flow out?

 

Again thanks fer yer help

 

graybeard

[Dick White]

You can set a hold on a manual kiln, but you must be there when the sitter drops. Lift the weight, press the white button to restart the kiln, and then very gently lower the weight again. The spring inside the cutoff switch (inside the little hole under the hinge of the drop arm where the little protrusion on the arm pokes through when it drops) is strong enough to hold the weight out at a slight angle when you gently rest it there, but not strong enough to withstand the momentum of the dropping weight when it falls as normal. Then you might need to turn the heating controls down a bit so the temperature doesn't continue rising. For that, you'll need to experiment to determine what setting works in your kiln.

[High Bridge Pottery]

To me it sounds like there could be something in the clay making the glaze flow away from that spot. What that could be I am not sure but if you have covered the same spots twice with glaze and both times it has vacated that area it sounds suspicious. I do remember reading shinos pinhole because of grog/sand in the clay. Is it a pinhole or more of something else?

[glazenerd]

Joel:

I have a working "theory" about clay compositions causing bare spots. As you noted, attempts were made to cover them to no avail. Alumina is used to control glaze flow; most being leached from the clay body itself, so there is no need to add it to the glaze. Crystalline glaze is very fluid by design and alumina is added to control glaze flow. Most bare spots end up in the 1/8 to 1/4 inch range; but I have had them as large as 1/2". Before someone states the glaze was too thin in application: crystalline glaze is applied at .75 grams per square inch compared to the usual .15 to .25 grams per square inch of normal glazes. So my theory is: the bare spots are spots were there are higher concentrations of alumina: or almost pure alumina that has not been blended. Clay bodies usually run 24 to 29% total alumina of batch weight. So having a high concentration of alumina in spots in not beyond the realm of possibilities.

From there my theory gets into electronegativity: negative and positive atomic charges repelling.. blah blah blah, I must say however that this cup of coffee tastes mighty good this morning.  

Nerd

[Denice]

I have several manual kilns and use a Skutt digital dual pryometer to do holds and slow cools.  I put a cone in that is a cone lower than I want and watch it closely at the end of the firing.  When it kicks off I manually turn it back on and the watch the temp on the screen and turn the kiln temperature knob up or down depending on what I want.  If you have a small kiln a single pryometer should be all you need.   I fire to C6,  I like to have the kiln turn off at C5 and then I fire it slowly to C6 and then just turn it off.     Denice

[neilestrick]

Friend, indeed.

 

A simple method to try without a pyrometer would be to turn the kiln back on immediately after the sitter drops, and put all the switches to medium for a while. It will slow down the cooling and give the pots a little more time at the high end. If you have a pyrometer, just turn the switches back and forth from medium to high to maintain temperature for as long as you need. I would think a 5-10 minute hold should suffice.

[graybeard]

Thanks to all! sounds like I'll be spending more time in the garage.

truth is I like it out there!!

 

G-beard