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Struggles Developing Clay Body From Local Clay

clay body formulation local clay clay/glaze interaction

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#21 Biglou13

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Posted 15 March 2014 - 07:53 PM

Have you tested vitrification/ absorption of fired un glaze piece?

The leaking possibly leans towards under vitrified body.

It could be other things.

Did you try same body with more spar, or replacing some if not all with nephsy.

Do you have some gold art it tends to pull a lot of "unknowns", finicky bodies togehter.

BIG congrats on slugging it out this far!!!!
It looks super good and promising.
Let me know if you need help testing for vitrification/absorption. Which I think should be your next in process.

Welcome to the "I made my own clay" club.
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#22 Rockhopper

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Posted 15 March 2014 - 09:46 PM

Have you tested vitrification/ absorption of fired un glaze piece?

The leaking possibly leans towards under vitrified body.

It could be other things.

Did you try same body with more spar, or replacing some if not all with nephsy.

 

 

My testing method so-far has been to mix small batches, and make two arch-shaped tiles from each.  Both tiles are bisqued at ^04, then fired at ^6 - one glazed & one un-glazed.  Have tested absorption (24hr soak + 2hr boil) on all un-glazed tiles that survived ^6 firing. 

 

My scale only weighs in whole grams, so not real precise on the relatively small tiles (35g dry) - but the previous batch, using 10% spar, showed the same weight after soak + boil as they did dry, indicating they absorbed less than 1g  ( < 2.8 % ).  I backed the spar off a bit for this batch, because the previous tiles had a bit of sheen to them, and I was afraid they might still be a bit over-fired.  This batch, with 8.5% spar, absorbed around 7% :(  so.. it looks I'll go back to 10% with the next batch.

 

Interesting you mention Gold-Art.  I did some tests a while back adding various combinations of GA + EPK, OM4, spar, & silica, and they were some of the worst results I've had (although all were still 60% 'raw' clay).  Following John B's comments to my previous post, I've adjusted to using 40-45% raw clay, and getting much better results - but haven't tried the GA again.

 

Aside from possibly increasing vitrification, what impact would GA have on plasticity ?  The current blend throws OK for me, but is not as smooth when wedging, etc, as the comercial bodies I've used.  I definitely don't want to make it less plastic.  What about adding bentonite ?  I know it's sometimes added to improve plasticity - would it also affect vitrification ?

 

 

BIG congrats on slugging it out this far!!!!
It looks super good and promising.
Let me know if you need help testing for vitrification/absorption. Which I think should be your next in process.

Welcome to the "I made my own clay" club.

 

Thanks!  I know there are plenty of folks out there scratching their heads, wondering why someone would put the time & effort into making their own clay when "store-bought" clay can be had "dirt cheap".  If I were trying to do this for a living, I would probably agree with them - but as a hobbyist, short of taking advanced college classes, I would never have learned this much about clay if I only used store-bought.  (And, besides, I think it's pretty cool that I'll soon be able to make useable mugs, etc. with the same creek clay I played with as a kid. :) )



#23 Biglou13

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Posted 16 March 2014 - 07:45 AM

GA is a stoneware body it seems to work with what I've thrown at at it. But I've read it's not as plastic as ball clay like om4. And both have lower melt than epk.

Anything you add changes vitrification, but bentonite is usually in very low percentage, usually less than 3%.

While I don't dig my own clay, epk is locally mined close enough to call it local, but not as local as "rockhpper's creek". But my pride level goes up having made it and keeping it somewhat local and indigenous. You'll also be surprised to how much street cred' local self made clay buys!
Caution big brother is watching.
The beige is blinding!!!!!!
The middle of the road is boring

The true sign of intelligence is not knowledge but imagination.
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#24 Rockhopper

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Posted 27 April 2014 - 05:04 PM

More testing completed... and more confused than ever..

 

I mixed four variations on the previous recipe - two with a little betonite added, and two with OM4 - trying to stop the leakage that occurred with the last test.  I made two identical pinch-pots from each, bisqued them all - then fired at ^6 one glazed, one un-glazed of each variation.  Then did 24hr soak + 2hr boil on the un-glazed

 

Mix (balance 'dug clay' approx 40%) Absorption EPK 35, Flint 15, Spar 8  +Bent 2 1.47% EPK 35, Flint 15, Spar 10  +Bent 2 1.47% EPK 34, OM4 3, Flint 15, Spar 8 5.88% EPK 32, OM4 3, Flint 15, Spar 10 5.88%

 

I filled all of the glazed pots with water & set them on a paper towel.  Two leaked, and two did not.  In less than an hour, there was water on the towel under two of them - and the water level in the pot was noticeably lower.  The other two had no water on the towel several hours later.

 

 

Herei's where I get confused:  The two that tested at < 1.5% absorption unglazed are the ones that leaked.  The pots that tested over 5% absorption unglzed are the ones that did not leak.  (No chance I mixed them up... each pair of test-pots was clearly marked as they were made.)

 

The glaze (floating blue) looks great on all four - no visible pinholes, crazing, etc.  I'm not really surprised, based on the absorption tests, that two of them leaked - but I would have expected it to be the two with the higher absorption.  Apparently I've still got a lot to learn.

 

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