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Green Dug Clay - What's It All About?

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Hi Guys, :)
while digging some clay (of the usually brown type) I unearthed a small deposit of grey clay.  Within the grey clay there was very small amount of distinctively green/grey clay with a green/grey stone in it.  Any folk got a clue or two to why this 'clay' was different?

I've kept separate from the other clay I dug that day and did the usual drying out and slaking down with this curious stuff.  It slaked down with no fuss, but once I'd blitzed it with hand-held food processor and screened the 'gritty bits', it 'almost' refused to settle out.  What settling there was was minimal and very slow to show.    Eventually I poured it onto a plaster bat and got a ball of clay (yay!). 

No idea if it's 'good to go' (I'm a nube when it come to dug clay) and there isn't enough to 'test' shrinkage etc; so has anyone got any ideas to what I got 'balled up?'.  I could mix it with some 'other' clay, but at the moment, I'm curious to this green/clay's make up and qualities.  Any best guesses welcome!
Ta mutchly,


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I'll bite. I think you have some organic material in there. I have a very dark clay, that when fired comes out pink in the bisque and brown at stoneware temps.

 Here's what you do;

1 Place said green clay[after completely drying it], into a bisqued bowl of a clay that you know will take that temperature.

2.Fire it up to bisque temps and see if the green colour remains.

3. If it does, you have a mineral like copper in the body.

4.If the colour burns out, then you had some kind of organic material like mould,fungus, a new life form.

5.If it melted into a liquid, you have a material for a slip glaze ala Albany slip.

6. Enjoy your new found wealth.


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We have the same thing with our Queenston Shale Clay deposits.  I've an old government document that talks about all the different types of clay in Ontario and even gives the chemical breakdown of each type.  This is a paragraph from the document:


Thin green bands parallel to the bedding, and

more rarely at right angles to it, have been

formed by the reducing action of acidic groundwaters.

Green Shale constitutes 5-25 percent of

most Queenston sections. Queenston Shale is

readily broken down by weathering, forming

ultimately a red clay soil with superior ceramic



The Queenston Shale is remarkably uni

form in chemical and mineral composition from

top to bottom of the formation; minor varia

tions have been pointed out in a previous

report (Guillet 1967, p.59). Lime content

is most variable, ranging 3-18 percent in brick

quarries of the Toronto-Hamilton area, re

flecting the proportion of green shale beds in

the sampled section. Green shale is harder, more

limy and less easily broken down by weathering.

Colour and porosity of the fired ware is there

fore dependent to a large extent on the propor

tion of green shale in the section. 


You can also see this in the Caledon Badlands:  Take a peek at the image.


Is it similar to this?

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My head hurts!  And digging the clay was so much fun!


Thanks for the replies folks. :)   The balls of clay feels a bit like plasticine and seems to keep itself to itself - doesn't transfer to the hands now it's a ball, like porcelain does - also looks glossy. 


You know, a question that was upper most when I truly started out with clay was 'what's the difference between earthenware and stoneware?'  The answer back then was one fires at a higher temp and both have their names on the bags.  Now the question is uppermost again, but this time it's how do I know if the clay I got will be 'earthenware' or 'stoneware' - what is the difference at the ground level?  I know  I'm gonna have to test fire to each temp, but what am I looking for with each fire and are there clues what might be what before I get to the kiln? :huh:


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Yeh, and you might want to fire measured test bars to note the shrinkage.

What price can you now put on these limited edition pots??

"Clay and Glazes forthe Potter" Daniel Rhodes, Pitman Publishing ISBN 027300218

has a good section on mining and preparing clay for potting.

A fairly old text but still useful.

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