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Natural clay


LHunt

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I dug clay out of our families “gold mining” creek in Calif. :) I have processed it down to almost pure clay body but with a tiny bit of fine sand..is this ok or do I need to rescreen it to get it all out?

Next question..is there a cone temp. That natural clay “generally” fires well at? And then what about the glaze temp? 

Thanks for all the input I appreciate it.

Leslie

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Most found natural clays around here (nor Cal) are lower firing and testing is the only way to find that temp range. I would start at cone 06-low fire and work up.

I have done what you are doing in collage and its a ton of work figure out what temp-its all trial and error.I developed a cone 10 body finally with that clay but I have to add lots of ingredients to make it not flux at lower temps. Good luck.

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at the opposite end of the spectrum relative to Mark's experience.

The yellow orange 'natural clay' from the bottom of my Texas pond still has bisque quality at cone 10.   Even the terra sigillata made from it fire to cone 10 is not melted.   I have to add lime and feldspar to get the clay to mature at cone 10.    I do use pond clay straight as a decorative slip over just about any commercial clay body at any firing temperature;  have also wedged that  clay into commercial porcelain to give the porcelain some character. 
  
I recommend you try it both ways,  with and without the fine sand.  See what happens.  
  
LT
 

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Mix exactly 100 grams of clay with 35 grams of water. Hand mix to a pliable ball. If it feels tacky, then you have a high plasticity clay that will handle cone 3-5 on its own. If it is dry and surface cracks form: you have a short clay that will need some help to mature and can handle higher temps. 

Next roll the entire lump into a thin strand, roughly 1/8th if possible. If it maintains its pliability: congrats you have a great cone 3-5 clay. If it begins to fall apart before the 1/8th bench mark: you will need to add at minimum 10% feldspar, 10% silica! and 10% ball clay. (oM4) works just fine. These additions will put you in the cone .6-8 range.

potters add fine sand to clay all the time: it makes clay life interesting. 

The only exception would be: does the fine clay have a green/grey hue? If so, it has high levels of calcium hectorite. If so, then cone temp goes way down. Hectorite is common in CA. 

T

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Gold and fireclay/kaolin tend to roll together as a geologic posse.  LT’s advice about lime and feldspar might be worth knowing in greater detail. But it could also just be alluvial secondary clay usable as terracotta or a glaze base (like albany slip).

Key pices of info are shrinkage rate and PCE (pyrometeic cone equivalent).  

Shrinkage rate is figured by forming  bars of a fixed length (10 cm makes the math easy), measuring wet, bone dry, and then a few different firing cones (say, 010, 06,03, 1, 3, 6 etc).

It is a good idea to fire these bars raised up on two chunks of kiln shelf.  This way, if they slump, bloat, or otherise deform or crack, you’ll be able to see it readily.

PCE is figured by firing a piece of clay formed into a “cone” of the orton pyrometric type and firing it until it bends like a cone fired to maturity.  If you have kaolin or fire clay, you won’t be able to get that cone to bend for anything at home (because the PCE of those materials will be like cone 30).  

If you do get the cone to bend.  Try firing a few test pieces of a fixed weight 4-5 cones lower.  Measure the %absorption by soaking a piece in room temp water for 24 hours, and one piece in boiling water for 5 mins.  Compare the weights of these soaked pieces to their dry weight.  The % increase is your % absorption.  

Thinga to be aware of are: dunting (could mean excess free silica turning to cristobalite)—LT’s suggestions are good here; Bloating meaning over fired (you also see this with a clay best used as earthenware); and drying cracks or cracking during firing from stress.  I find this happens in bodies with a high water of plasticity.

Once you know all this, you have enough information to determine what to do with this clay.  Or if you find it’s not workable, glazes can be fun.

Good luck and post results!

Oh!  One last thing!  Dry a chunk of the clay and chuck it in some vinegar.  If it fizzes from specific points, you have limestone pieces in the clay which will cause lime pops once fired.  This is a fault, but not necessarily a fatal one.  Some mexican tiles exhibit lime pops, as do many SW Ontario bricks of the yellow type.If the clay all fizzes, you have marl, which is workable as an earthenware with some work.  Delftware was made with marl historically.

 

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1 hour ago, Magnolia Mud Research said:

at the opposite end of the spectrum relative to Mark's experience.

The yellow orange 'natural clay' from the bottom of my Texas pond still has bisque quality at cone 10.   Even the terra sigillata made from it fire to cone 10 is not melted.   I have to add lime and feldspar to get the clay to mature at cone 10.    I do use pond clay straight as a decorative slip over just about any commercial clay body at any firing temperature;  have also wedged that  clay into commercial porcelain to give the porcelain some character. 
  
I recommend you try it both ways,  with and without the fine sand.  See what happens.  
  
LT
 

I added my location to that statement as location is key to what temp range it can be. Wish We had high fire clay naturally around here,

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2 hours ago, Magnolia Mud Research said:

at the opposite end of the spectrum relative to Mark's experience.

The yellow orange 'natural clay' from the bottom of my Texas pond still has bisque quality at cone 10.   Even the terra sigillata made from it fire to cone 10 is not melted.   I have to add lime and feldspar to get the clay to mature at cone 10.    I do use pond clay straight as a decorative slip over just about any commercial clay body at any firing temperature;  have also wedged that  clay into commercial porcelain to give the porcelain some character. 
  
I recommend you try it both ways,  with and without the fine sand.  See what happens.  
  
LT
 

Yeh clay / slip from bottom of dam can be left forever it seems without settling out. End of summer finds it as a shiny layer on top of clay  "tiles"

Makes a bright orange decorative slip and nice terra sig

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Wow..THANK YOU! What a huge amount of fantastic info. It’s such a funny story.. gold panning I was teaching my sig. other how to ..I said..”dig up that clay over there..the gold sticks in it...” and I went ..gasp..clay! Long story short..hauling buckets full up the hill to bring it home..I will chronicle the results..

 

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