Absorption rate.

[mwar15]

I could be wrong but,… I get asked “ is your pot fully vitrified.” I think people are worried about pots cracking from freeze/ thaw. I think the engineering and design of a pot play into this. So I calculated the absorption rate of the clay I use Unglazed.

I took a test fired to cone 6= 1162gr
Boiled for 2 hrs =1210gr


boiled weight-dry weight/dry weight*100
4.13%

I have been told my clay is fully vitrified. I haven’t had pots crack from winter freeze/thaw, but I live in a mild winter location.

something to think about as your pick clays.

 

[JeffS73]

In the UK, no manufacturers publish absorption rates for their clays, so I test all the clay I use, I think it's good practice. German clay is a godsend, all aspects tested and documented, with graphs etc. English Clays look nice, so for me, they're worth a try, just test.

I use the methods outlined on digitalfire:
https://digitalfire.com/test/shab

A point to consider is that fully vitrified, at 0% absorption, you have glass, which is less strong than ceramic at 1-3%. A pot should be able to take a knock or two. See Hamer and Hamer for more info.

Also, as clay becomes more vitrified, it begins to look more glassy. This is a key trade off for unglazed pots, IMHO.

I found a nice English clay, called red stoneware, but absorption at 14%, no good. With 20% added nepheline syenite I can get it to 3%, usable, but it lost a lot in appearance.

 

[mwar15]

In the UK, no manufacturers publish absorption rates for their clays, so I test all the clay I use, I think it's good practice. German clay is a godsend, all aspects tested and documented, with graphs etc. English Clays look nice, so for me, they're worth a try, just test.

I use the methods outlined on digitalfire:
https://digitalfire.com/test/shab

A point to consider is that fully vitrified, at 0% absorption, you have glass, which is less strong than ceramic at 1-3%. A pot should be able to take a knock or two. See Hamer and Hamer for more info.

Also, as clay becomes more vitrified, it begins to look more glassy. This is a key trade off for unglazed pots, IMHO.

I found a nice English clay, called red stoneware, but absorption at 14%, no good. With 20% added nepheline syenite I can get it to 3%, usable, but it lost a lot in appearance.

I think that is the trade off. A good looking clay body when unglazed VS shrinkage and absorption VS structural integrity

 

[penumbra]

A clay that is fully vitrified can still be pretty absorbent. Fortunately the clays I use here are all stated. For outdoors use I use clays that have an absorption rate of 2% or less. The clay I use for my Stone Age pots has an absorption rate of . 25%. It is a very very tough clay partly because of the heavy grog content. It is actually much stronger than many clays I have used with higher absorption rates. It is also much stronger than clays I have fired to higher temperatures, which can be more brittle. I have never seen a stoneware clay with 14% absorption when vitrified. About the highest I have even seen is around 6%, and I will not use it for outdoor ware.

 

[Sansokuu]

Honestly, unless the pot has a crack or pools water, the clay absorption is a bit of a red herring to more likely reasons for a pot to crack, which is the shape of the pot (shouldered/rounded pots vs flared/more straight sided) and how wet/broken down the medium is. People asking for vitrification of home made pots also seems less necessary unless the potter used terracotta, though I could see why they would as a lot of mass produced pots are low fired for saving both time and money.

Here’s a good article on it, which I found on Red Tail Pottery’s faq: http://whistlingfishpottery.weebly.com/musings-of-a-mad-potter/frost-proof-pottery-question

It’s a nice thought to get absorption rate as low as possible - my favorite clay company, Laguna based out of Socal with both west and east coast distribution lists their absorption rates on their clay body descriptions. Some of their porcelains rate below 1% absorption though they are practically glassy at that point.

 

[mwar15]

I agree. Great article!