: Celia UK - Viewing Profile

Celia UK, on 22 December 2012 - 11:27 AM, said:

Ah yes, that may be a bit trickier to control since you would not be able to isolate a single component of the prepared powder, but it couldn't hurt to do a few 100gram test batches with different concentrations (say .5%, 1%, and 3%) of a frit that contains borate added to your glaze powder. It is a good thing that you get the glaze in powdered form, because if it were premixed with water then it would be much harder to consistently add the same amount of frit to each glaze batch (if the testing turns out).

I have definitely found that crazing will happen when there is too thick of a glaze application. Using just clear glaze really doesn't take much to make a nice glossy seal. If you take a credit card and scrape a line in the glaze you should see that the thickness on the piece is about the width of credit card or even a touch less (you can then rub your finger back over the scrape to fill the glaze back in). Also, if you thin it down it would definitely help glazing the piercings. (I too am very much into pierced designs) I find if the hole has filled in as I pull the piece up from the dip, if I blow gently into the opening it will break the surface tension and possibly clear it. Otherwise if it stays closed, depending on the shape, i can use a small drill bit to hand "drill" the hole out without completely chipping away the dried glaze. You can take your finger and gently rub over the runs to "sand" them away so to speak, or you can use actual sand paper to gently grind them down. Just make sure to try to keep the dust down (wear a mask).

I was taught that when you stick your hand in the glaze and pull it out there should only be a thin layer still stuck to your fingers, if it's thick and sticks like gravy to the back of a spoon it's too thick. There is also a way to make a density meter with a block of wood and a weight so that you know the glaze is the same thickness every time you make it.
Basically you attach the weight to the end of the stick and place it in a bucket of plain water so that part of the stick is floating above the surface. Mark where the water level is on the stick.
Next, place it in the bucket of well mixed glaze that you know has been combined with the right amount of water (you've tested how well it covers the bisque and possibly fired to see if it crazes) and mark where the glaze level falls on the stick (it should be lower than the water mark).
Then, the next time you mix that glaze you can drop the stick in the bucket in between intervals of adding water to the powder until the stick comes to rest at the glaze level line. You should end up having the same density glaze every time. As with nearly everything, this was not my original idea. I believe I got it out of a Ceramics Monthly magazine actually.

As far as firing the bisque to a higher temp then glaze firing, I've honestly never done so since I fire to cone 9/10 and the clay would never accept the glaze at that point since it would be completely vitrified. But I am curious if it helps with crazing at lower temps since it is much more economically feasible to fire lower.

Best

Shannon