Firing/Density

[chrl42]

There is another thing I am curious about. How does the rate that water dries off the clay relates to the firing? Does that ties in directly with the porosity of the clay?

It might be obvious but I am...curious as I notice that zisha dries at different rates.

It's true. Different Yixing clay has a different porosity rate...but it's really complicated subject.

It clearly relates to the temperature of firing..the less temperature the more porosity. But many potters and collectors think less temperature as less quality...so it's a dilemma.

I've heard, that if a clay contains more 'sand', the clay has the stable porosity even with high temperature to fire...it's not an easy subject.

The porosity of Yixing clay gives the clay a quality of breathing, more sanitized...and high temp of firing gives more clean patina and better color, and better brewing ability....but too sandy clays are not easy to work with for potter, they lack plasticity.

So modern potters try to solve that contradiction by mixing clay, hand-grinding clay...as far as I've known.

[paul haigh]

I was in South Carolina recently, near a town famous for pre-Civil War (slave made) pottery. A friend there gave me local clay that they dug- very gritty, interesting inclusions, a lot of sand etc. It threw on the wheel like a dream, and seems to have low shrinkage, making it great for the sculptural work that I have been playing with.

"Sand" (or grog- which is fired clay that is ground and added back to a clay body) can change plasticity, but I wouldn't say that it's a hindrance for anyone with skill. So many chawan are made with the nastiest clay (with sand/rocks in it). I have been throwing 10-20# pots with heavily grogged sculptural clay recently because I am using them as the basis for sculptural pieces (face jugs). Hasn't been a major issue.

(Using Sheffield S14, and home-modified clays with local sands)

[chrl42]

I was in South Carolina recently, near a town famous for pre-Civil War (slave made) pottery. A friend there gave me local clay that they dug- very gritty, interesting inclusions, a lot of sand etc. It threw on the wheel like a dream, and seems to have low shrinkage, making it great for the sculptural work that I have been playing with.

"Sand" (or grog- which is fired clay that is ground and added back to a clay body) can change plasticity, but I wouldn't say that it's a hindrance for anyone with skill. So many chawan are made with the nastiest clay (with sand/rocks in it). I have been throwing 10-20# pots with heavily grogged sculptural clay recently because I am using them as the basis for sculptural pieces (face jugs). Hasn't been a major issue.

(Using Sheffield S14, and home-modified clays with local sands)

Maybe it's a different subject for Yixing clay..I tried to remove my personal opinions and follow Yixing potters' opinion at best.

It doesn't have to with 'grog (熟砂)' at all, adding grogs are very common when making Zhuni pots..adding it makes for a higher success rate (from cracking or bursting in a kiln) because Zhuni pots shrinks like hell.

Sandy clay I mean, doesn't have to do with nasty clays, premise is when the clay is pure at best..or what they say silica amounts...but it's very complicated subject..because Yixing clay is naturally mixed clay and unexpected result could happen anytime..

[paul haigh]

Maybe it's a different subject for Yixing clay..I tried to remove my personal opinions and follow Yixing potters' opinion at best.

It doesn't have to with 'grog (熟砂)' at all, adding grogs are very common when making Zhuni pots..adding it makes for a higher success rate (from cracking or bursting in a kiln) because Zhuni pots shrinks like hell.

Sandy clay I mean, doesn't have to do with nasty clays, premise is when the clay is pure at best..or what they say silica amounts...but it's very complicated subject..because Yixing clay is naturally mixed clay and unexpected result could happen anytime..

Adding sand doesn't change the silica amount per se, as it is not homogeneous. Back to the original statement- adding sand or grog will have similar effects- it changes the plasticity, but that's generally not a big deal.

Maybe I'm not following the conversation well enough here.

[chrl42]

Maybe it's a different subject for Yixing clay..I tried to remove my personal opinions and follow Yixing potters' opinion at best.

It doesn't have to with 'grog (熟砂)' at all, adding grogs are very common when making Zhuni pots..adding it makes for a higher success rate (from cracking or bursting in a kiln) because Zhuni pots shrinks like hell.

Sandy clay I mean, doesn't have to do with nasty clays, premise is when the clay is pure at best..or what they say silica amounts...but it's very complicated subject..because Yixing clay is naturally mixed clay and unexpected result could happen anytime..

Adding sand doesn't change the silica amount per se, as it is not homogeneous. Back to the original statement- adding sand or grog will have similar effects- it changes the plasticity, but that's generally not a big deal.

Maybe I'm not following the conversation well enough here.

I hope it'd be simple enough.

I mean, every Yixing clay has mineral composition data..Di Cao Qing clay has higher iron rate than Zini..Zhuni has higher iron rate than Hongni etc. Ben Shan Lv Ni scores the highest rate of silica...Ben Shan Lv Ni a clay said to be 'not able to make without mixing' it lacks plasticity..this clay is very sandy at best. Hongni has more silica than Zhuni, Hongni is said to be sandy red clay...many Hongni lacks plasticity as well...Factory-1 added Baini to Hongni back then...old Qing Shui Ni lacks it. Di Cao Qing is also what they call of 'sandy' group...potters say Di Cao Qing clay needs to be stored for a decade..then the plasticity will be increased...

[paul haigh]

Sand and grog both reduce plasticity. They both increase porosity. Those are both good general rules. For modern production- wet process/slake it, add a weak acid (like Epsom salts), vacuum deair pug it- now it acts like aged clay.

I guess I'm not following the discussion beyond that. I'm a simple guy

edit: if someone had access to a clay description, then they might predict the porosity/density of a piece sight-unseen, but this is a case of theory guides and experiment decides. I'm not qualified to speak about the qualities of any particular clay of the types discussed.

[betta]

Sand and grog both reduce plasticity. They both increase porosity. Those are both good general rules. For modern production- wet process/slake it, add a weak acid (like Epsom salts), vacuum deair pug it- now it acts like aged clay.

I guess I'm not following the discussion beyond that. I'm a simple guy

edit: if someone had access to a clay description, then they might predict the porosity/density of a piece sight-unseen, but this is a case of theory guides and experiment decides. I'm not qualified to speak about the qualities of any particular clay of the types discussed.

Paul, does the porosity of the material affects its durability? As liquid penetrates through the pore, it also exerts capillary force to the material.
This is the same force that causes wet wood cracks and wall exfoliated.
I am interested to know whether a porous ceramic or stoneware teaware can develop cracks over a long time use (wetting and drying, temperature swing hot and cold). Thanks for the info.