cratering problem

[Robert Coyle]

This is a modification of a glaze from Glazes Cone 6 by M Bailey
 It gives a real nice matt blue break where thick and  matt green where thin.
 My problem is that it always looks fine on the test tiles I fire but some of the  pots I fire show craters (not pin holes) and fish eyes, while others, from the same kiln load,  look fine. A second firing sometimes solves the problem but not always. Any thoughts?
             
             Neph Sy     69
             GB              9
             Ball Clay       8
             Whiting        5
             Silica           5
             Rutile           2
             Cobalt Carb   1
             Titanium O2  1

[Robert Coyle]

 Thanks, Brian.

I probably need to re-formulate in a systematic way. I'll start with the GB and instead of running test tiles , I'll make some mugs or something so I have a better chance of catching it, since it never showed up on the test tiles.

[Robert Coyle]

I always do 300 gram test  batches. This is a convenient weight for the double pan balance I use. My fear is that the larger batches I make are less accurate than the test batches, since I use a digital postal scale which is only accurate to about 5 grams . I try to compensate for the built in inaccuracy of the postal scale by weighing the smaller amounts of ingredients on the double pan that has a 1/10 gram accuracy.

It seems like any real glaze you would want to use should be able to tolerate a couple of percent slop in the  accuracy of the formulation... at least for the major components.

I am not a production potter so I make up only about ten pounds of glaze at a time. I also mix the new batch in with anything remaining of the old batch. I don't know if this is a good idea or not, but I hate to waste it. I run a test tile on the final mix to make sure it fires alright.

I brush the glaze on rather than dipping. The glaze breaks blue where thick, and I try to encourage this by brushing on thicker in some areas.