Potters & Sculptors - Making Rock from Mud
I've been potting for about 4 months, mainly wheel throwing. I've had successes and failures but apart from my first glaze firing, when I stupidly bonded the rim of a few pots to my kiln shelves, I've considered that my firing has gone reasonably well.... until now, as I've realised I have some porosity problems.
This has come to light on lifting a couple of pots containing plants, only to find some damp rings underneath. The pots are not leaking as such, but moisture is certainly leaching through.
I'm using a lightly grogged buff stoneware with a firing range of 1120C - 1280C (2048 - 2336F) and am bisque firing to cone 04(*) and glaze firing to cone 5.5(*). I'm using Amaco Potters Choice liquid glazes and am typically applying 4 coats internally, irrespective of what I'm doing on the outside. I have also experimented with some ash glazes made from 50% local mud, 20% ash from my wood burner and 30% nepheline syenite. I have used (*) against the cone numbers as I haven't yet purchased any witness cones, which is probably one of the first things I should have done.
That being the case I initially followed my kiln manufacturers instructions for a reasonably thick wall (just to be on the safe side) the glaze firing cycle being as follows:
100C (212F) per hour to 600C (1112F)
220C (414F) per hour to 1203C (2197F)
Soak 10 minutes
In more recent firings, containing both Ash and Amaco glazes, I have modified the second ramp/dwell to be:
200C (392F) per hour to 1203C (2197F)
Soak 30 minutes
I went for the lower rate/longer dwell principally for the ash glaze, but also as some pots had very slight pinholing and I thought the slower ramp might help. However, I don't suffer pinholing across the board and think that perhaps where I do this is down to surface finish and rushed glazing prep.
I could understand the porosity if I'm not going to a sufficiently high cone to fully vitrify the clay but the Amaco glazes are cone 5-6 and I'm fairly sure I'm achieving cone 5, so even if the clay is slightly porous my glaze shouldn't be. So I'm kind of struggling to understand whats going on.
I was initially a little bit surprised by the kiln manufacturers recommendation regarding the second ramp rate,considering that Orton cone temperatures are based on a heating rate of ~60C (140F) for the last 60 minutes of heating. Makes me wonder whether I should have gone for a programmer with more than 2 ramps.
Would my problems be solved just by pushing a bit higher, say to cone 6,with a slower second ramp?
Any advice gratefully received.
I would try exactly what you thought of going to cone 6 with the second ramp. Also are you using witness cones? Good luck, look forward to hearing how it goes.
Fire test tiles of each clay and glaze.
Some vendors, like Laguna Clay, publish the porosity for each of their clay bodies.
Laguna Hagi Porcelain, fired to cone 5 without glaze, absorbs 0.5% water by weight.
Laguna Speckled Buff, fired to cone 5 without glaze, absorbs 3.0% water by weight.
Max's Paper Clay, fired to cone 5 without glaze, absorbs 11.0% water by weight. Fired to cone 10 absorption is still 5%. Very sponge-like.
After applying and firing a glaze which is glassy enough, even Max's Paper Clay becomes water tight if there are no fractures. But many ceramicists don't like applying that much shiny glass to their artwork.
While these three glazes are not - Dry Pumpkin and Stonehenge Pike's Oatmeal. Guessing isn't the same as testing.
Thanks for the feedback, I'm typically quite methodical but having had what I thought was a success I've pretty much stuck with a method with minimal variation, whereas I probably should have experimented some more. All down to wanting to create finished pots I guess. I've made vertical test tiles for the ash glazes, but I'll start testing more routinely, even with commercial glazes. For starters I think I'll make some small dished tiles to test for porosity. I'll certainly go out and buy some cones.
When you get your cones remember to give yourself a zone, if you are shooting for cone 6 I would place cones 5-7 on low, medium and high shelves. That will give you empirical evidence of what is going on in each zone. I am not a big fan of flat test tiles since that only provides limited color information. I prefer to save to use cracked bowls or other defective pieces from the same clay body. That way I can see how the glaze will run etc...
Good luck, I can't wait to see some of your work.