Wide firing range claybodies for functional work.

[birdypotter]

I recently pulled out my first mugs from a cone 6 firing. I used Draycott Stoneware from Potclays, UK and fire in oxidation to 1050 degrees c slow bisque and 1222 in a glaze firing, slow cooling to achieve matte surfaces with my glazes. I glazed the inside of the mug with a commercial transparent glaze from Scarva pottery supplies (so unsure of the CTE) and the outside of the mug with a satin matte glaze that has a CTE of 6.5. After a week of test using the mug (pouring boiling water I know will have exacerbated the problem), 2 big hairline cracks appeared on the inside of the mug. I smashed the mug apart to check the throwing thickness to see if there were any thin spots in the walls that may have been put under stress and cracked but the thickness seems to be pretty even and consistent.

I know potentially the problem could be mismatching CTEs of the liner glaze and the outer glaze and so I am now mixing up my own transparent glossy liner glazes to regain control over that process - but my question is how closely matched do the inner and outer glazes have to be in terms of CTE? Could the problem be that they don't match the body? and looking at my firing temps and process - could the mug be under fired and allowing water to seep in through the unglazed footless bottom surface and placing inner stress on the clay? Am I missing something?

Any advice greatly appreciated!

[Min]

A few thoughts, I looked up your clay and it's rated as having a 1200-1300C firing range. By firing it to 1222C (approx cone 6) it won't be mature. 1300C is approx cone 10-11 so there will more than likely be absorbency issues. Whether or not this is the cause of the crack is debatable but I doubt it. Run an absorbency test with the clay and see what figure you get to verify this. To do a quick but not accurate test you can also put a drop of food colouring on an unglazed part of the pot and leave it for a couple hours then rinse it off. If the claybody is stained it's porous but obviously this won't tell you how porous it is. 

Second issue is CTE figures for slow cooled matte glazes are not accurate. During the cooling some of the glaze ingredients precipitate out of the glaze and form micro crystals. If it's a high calcium glaze for example the mattness will be from calcium micro crystals therefore the calcium is not part of a complete melt therefore the calculated CTE figure will be off. 

The mugs should be able to withstand boiling water poured into them. For durable functional work they should be able to withstand taking a frozen pot and pouring boiling water into them.

Analyzing cracks from Frank Hamer has really good descriptions of cracks and their causes. Have a read through that and see if crack M looks familiar.

 

[birdypotter]

Thanks so much for your response Min. The knowledge about inaccurate CTE figures for slow cooled mattes is particularly helpful to know.

I requested the clay data sheet from the suppliers and it said that the vitrification temp of my clay is 1240-1270, so cone 6 is possibly slightly low and perhaps I need to consider raising the temperature and seeing what effect that would have on my glazes.

I did a brief weight test by soaking a pot with the unglazed foot in a bucket of water for a couple of hours, weighing before and after and found it took on 1.6% of its weight. Unsure whether that's about average or too much but the website link you gave me said between 1-3% is fine. I will need to redo this test to get more consistent results; some sources suggest boiling the piece in water rather than just soaking? I also froze the mugs overnight and then immediately poured boiling water in them, dried them off and coated the insides with ink to reveal any cracks. I have 2 different base glazes and the crack appeared on the mug with the same outer glaze as pictured above but not on the others which leads me to think that particular satin glaze is a poor fit to the body. 

Crack M looks pretty similar and looking at the section on dunting in the book, it suggested that a crack that happens a couple of weeks after is a glaze fit issue. I think shelving that particular glaze and rigorously testing the other might be the way forward!

[neilestrick]

If it came out 1.6% just from the unglazed foot, it would be much higher than that if the entire pot was unglazed.

[birdypotter]

That's a good point, thanks, I will retest an unglazed bar and see what the results are.

[Min]

5 minutes ago, birdypotter said:

That's a good point, thanks, I will retest an unglazed bar and see what the results are.

Make up a few and scatter them around in the kiln so you can get an average figure as kilns can have hot and cool spots. Once you've got the claybody sorted out that you are going to use then I would suggest making up some thin walled cylinders and using those to test glaze fit for dunting rather than mugs etc. They can just be quickly made, slab or thrown, then if the glazes don't fit you haven't wasted any mugs. 

[birdypotter]

I've repeated the test just now with an unglazed bar and it has come out at 4.4% so not disastrous but higher than I would like. I'll repeat the test as you suggested min to get an average but thinking perhaps I should raise my firing temp a little to take it closer to vitrification? My glazes that I have developed are cone 6 so I'm concerned 1240 (the beginning of vitrification for this particular clay) would be too high for the glazes. Perhaps 1230 and retest for absorbency to see if I can get that 4.4 to drop sufficiently?

 

Making up some basic cylinders might be a good idea too!
Thanks for the advice so far, very much appreciated.

[Min]

1 hour ago, birdypotter said:

I've repeated the test just now with an unglazed bar and it has come out at 4.4% so not disastrous but higher than I would like.

Hmmmm, need to soak it for longer than an hour so two. Really check it by weighing a bar after it comes from the kiln then boil it (fully submerged) for 5 hours then let it cool in the water for 24 hours. Wipe it dry then weigh it. If your absorbancy was 4.4% after a short period of time it's definitely going to be higher than that with the boil then long soak. This test checks for both open and closed porosity, might be a bit of overkill but it will give you a worst case scenario. 

 

[birdypotter]

3 hours ago, Min said:

If your absorbancy was 4.4% after a short period of time it's definitely going to be higher than that with the boil then long soak. 

 

I'll definitely do this test to be doubly sure but if it is 4.4% now then is it safe to assume at this stage that with more rigorous testing it'll be even higher and will be too absorbent for durable everyday use? Is raising the firing temp the answer?

[Min]

1 hour ago, birdypotter said:

I'll definitely do this test to be doubly sure but if it is 4.4% now then is it safe to assume at this stage that with more rigorous testing it'll be even higher and will be too absorbent for durable everyday use? Is raising the firing temp the answer?

For this claybody to get it mature enough to be durable without leakage concerns then yes raising the firing temp would be the way I would go.

Other option would be to change claybodies if you want to stay with your current glazes. I don't know what you have available to you over on your side of the pond but a quick look at the Potclays site shows a white earthenware that is rated to go to 1220C which is approx cone 6. I have no idea how porous that would be but it might be worth looking into if you want to fire to ^6, your supplier should be able to supply you with some info for this clay or what else is available to you.

Over here lowfire and earthenware is generally in the ^04 range but it seems some of yours can go to ^6. Firing to ^6 is much easier on the kiln elements than going upwards toward ^9 / 10 that your current clay might need firing to in order for it to be tight enough not to leak. Glazes cannot be relied upon to make a watertight seal on the clay so optimally you want the clay to be tight and not weep even when unglazed. 

[birdypotter]

Thanks @Min, working alone without help and learning you go can be super frustrating so really value the advice.

Difficult to choose a clay that I know will vitrify, I was looking at earthstone es5 from Scarva which lists its firing range as 1200-1290 with it maturing at 1200 - which for a clay with such a wide firing range seems quite low to be fully matured. Can't find any further info on any other data sheets for that particular clay currently.

Looking at the data sheet for all of Potclays options, unless I went to semi porcelain or dropped down to earthenware then the Draycott stoneware has one of the lowest starting vitrification point of all the stonewares with 1240. I want to try and work with the clay I have currently to see if my glazes survive that push in temperature - though 10-20 degrees centigrade increase could be dramatic...but optimistically that'll be the first port of call before I think about completely switching up the body. Was reluctant to move to earthenware as I was under the impression that earthenware wasn't as durable as stoneware and more porous, is that a misconception?

I'll do the aforementioned bar tests and experiment with slightly higher firing temps and post an update here if I remember incase the results are helpful for anybody else!

 

 

[birdypotter]

An update for anybody using draycott stoneware or Earthstone 5 that might find this helpful:

I tested my original clay (draycott sw from Potclays) at 1230 and 1240 and did a full absorption test on 10 bars (boiled for 5 hours and soaked for 19) and the absorption % came back at 3.9 for 1230 and 3.5 for 1240. My glazes began to change at 1240 and 3.9 at 1230 was still too much absorption for my liking (was looking for between 1-3). To use this clay I would have had to go much higher and would have had to change glazes in the process. 

I chose a new clay - earthstone 5 from scarva for UK potters - and the same test for absorption for 1230  came out at 2.1% and 1.2% at 1240, which is much better. 

I also tested earthstone 10 extra smooth which has a lower vitrification range and came out at 0.3% absorption at 1230 and 0.2% at 1240....so much more vitrified at these temps than the other two. 

So, a change of clay is in order to be able to make durable microwave safe dinnerware at 1220-1230. Hope this is of use to somebody!