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Black Clay Advice


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I suggest since you are new to ceramics working with black clay is like starting a hike with a monkey on your back . No need to do it really-just get another bag of easier to work with clay and save the black clay until you have some experience under your belt.

My 2 cents as ceramics is a challenge enough with the easier materials.

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  • 3 weeks later...

Got piles of ugly failed Cassius test tiles but each one is a successful learning experience.

Also testing a clay called Oregon Brown. It takes two firings to achieve the darkest color: near-black. Easier to work with than Cassius but just as fussy to glaze.

Why I'm still at it:

The same pieces in off-white, buff, -dependable, etc; clays, just don't have the drama of the dark pieces. Yes bloating, occasional cracking, 'eating' glazes, but the pieces that do turn out are just gorgeous. Unglazed Cassius pieces look unusual and finished. I enjoy the bizarre things it does, like resulting in colorful mossy or wrinkled glazed surfaces. With sculpture or modern jewelry pieces you can get some really cool effects. Using brightly colored underglaze over white slip-painted areas creates dramatic jewel tones that stand out beautifully against  black left showing. To my eyes it's a lot like painting on a black velvet canvas. 

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Just wanted to share my experience with Cassius Basaltic.  Most glazes end up with pin holing, bloating, or just ugly on the surface on anything Cassius I do.  However, I've found that our studio's "licorice" and "blue hare's fur" work great on the surface as do most underglazes after bisque (not sure about greenware, haven't tested applying at that stage).  Here are some shots of underglazed vases with licorice inside and a cup coated in BHF.

Cassius Vases with Underglaze

Cassius Mug with Blue Hare's Fur

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Did I mention that the raw black clay (PSH) attracts a magnet? Hilarious- a leather hard item with a magnet sticking on the side- so... the iron is Magnetite or Fe3O4. After firing to cone 6, it turns brown - a very dark brown and is no longer magnetic so the iron is altered to hematite Fe2O3. 

My kilns are old fashioned- electric with a kiln sitter and gas caternary arch- no computers here except this laptop! So maybe one day when I have time, I will try bisquing to a higher temperature as I have no "hold" capabilities. 

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for some cultures though black clay is the norm isnt it? i have a particular fondness for danish pottery. esp. the potters island. all the photos i have seen and movies - (yeah i look at their dirt) seems like they have black volcanic soil. most of their pottery is dark clay. 

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"Just wanted to share my experience with Cassius Basaltic. Most glazes end up with pin holing, bloating, or just ugly on the surface on anything Cassius I do. "

 

Try firing Cassius to cone 5, not cone 6. Also, avoid stacking during bisque so all of the surface areas have room to outgas.

 

You can also reduce Cassius to a slip, then apply it to white or other clays. You get the black to work glazes with, but not the clay body issues.

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  • 2 years later...
On 1/5/2017 at 11:00 PM, bciskepottery said:


hi there, when yousay hot bisque can you be more specific in temp? Captain newbie here  

 

 

With dark clays, a good, hot bisque is needed . . . with a hold at top temperature to help burn off gases and impurities that can lead to bloating. Avoid dense stacking of the items in bisque -- give them breathing room.

 

You did not mention what cone you are firing to. Assuming it is cone 6, when glaze firing, try firing to cone 5 and add a 20 minute hold rather than firing up to cone 6 -- you might reduce the likelihood of bloating. The hold will bring your kiln heat work up to cone 6.

 

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