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Wednesday, 18 February 2009

What is your specific gravity?

All day yesterday, I embarked on a journey of better understanding of my glazes. For so long now, three of my long-time glazes have managed to confuse me with their "random" outcomes from many a gloss firing.
As many potters know, variations in fired glaze outcome could be one of numerous factors:

  • firing schedule - speed of climb, soak time, temp range, cooling ramp time
  • consistancy of glazes
  • raw materials in glaze - variations in where certain materials were mined, etc...
  • clay you are using
  • the pack of the kiln
  • how lucky you are feeling on the day, and many more....
I'm working off the assumption (for the moment and living dangerously here!) that my raw materials are pretty consistant; my clay has been constant for 3 years; I have been aware of the kiln pack issue so have been and continue to try and pack in a very similar fashion (density and arrangement of pots/shelves) every time I fire; and I'm feeling lucky.

So, that leaves firing schedule and glaze consistancy for me to work with and sort out.

This week, I'm tackling glaze consistancy.

After reading hundreds of posts on my favorite potter's resource website regarding hydrometers and the overwhelming lack of confidence in being able to obtain accurate figures to determine the specific gravity of a glaze, I went with what many a potter seem to feel is a good, tried and tested easy method of getting that figure in a more accurate way.

I took each glaze I wanted specific gravities for and did the following steps:
  1. took a standard measuring cup and placed it on a digital scale, tared it to zero.
  2. I filled it with water until the scale read 100g. I marked on the cup where this point was. t
  3. I then took my glaze batch and syphoned off all the excess water and measured in oz/ml
  4. took half the total excess water, added it back into the glaze batch and mixed well. once well mixed, I poured it into my standard cup to the marked level and weighed it. the total, divided by 100 gave me my specific gravity of my glaze. By adding only 1/2 the water into my glaze batch, I labelled this test as my THICK version of the glaze.
  5. I then proceeded to brush the glaze on in three progressive layers onto one bowl; then dipped in 3 progressive layers on another bowl
  6. To obtain a MEDIUM thickness test, I then added the remaining half of the excess water to the glaze batch, mixed well and measured it again. This became my specific gravity for the MEDIUM version of the glaze. I proceeded with the same application tests as the THICK glaze.
  7. Finally, to obtain a THIN version of the glaze, I added additional water to the batch, equal to half the original excess water amount (ie, if total excess water in the beginning was 56oz, I added 28oz for THICK test, additional 28 oz for MEDIUM and another 28oz for THIN). I mixed well, weighed it again to the marked line in the standard measuring cup and got my specific gravity for THIN. I proceeded with same application tests as other consistancies.
So, for each glaze, I got 6 different tests : THICK, MEDIUM and THIN consistancies all applied with brush or dip.

I'm hoping my increments with the water were of significant difference to the specific gravities of the glaze to see a real difference between the three consistancies. If not, I'm gonna have to do it all over with a different approach to the water amounts! Seeing that I'm making this up as I go along, based on various bits I have been reading, I won't be suprised if all this testing won't bring all the results I am hoping for. But, it is a learning curve, right?

At the end of the day, I tested 7 glazes this way and was able to view the fruits of my labour, even though they don't look that impressive!!!

I'm loading the kiln as soon as I am done posting this! Boy, I hope this produces something worthwhile to see!


  1. Jami,
    I love the blue speckled finish. Very cool. Keep up the good work. So what do you do on the 3 days you work for you "bread and butter?"

  2. Thanks Rachael!
    I work as an admin assistant in central london doing the whole office thang...way better money unfortunately than working part-time in the arts sector (full time even is bad)...