Varied Perlite inclusion results

[Charles]

Here are photos of a Perlite facade on a vase I made last spring, and a piece I found yesterday called Organ from Louise Gregg of Australia.  I found a site for her, but it hasn't been updated since 2012, and I can't find any other web references for her, so she might not be currently active in the arts.

 

https://www.flickr.com/photos/29724621@N03/sets/72157701256082844

 

I don't understand how we could have achieved such different results - unless perhaps Aussie Perlite is of a different composition than American Perlite.

 

However, I've seen variations in how Perlite performs.  The first time I used it on a cup, it produced quite sharp edges, and the studio manager said it approached being dangerous to handle.  There are some rough edges on the vase, but I can run my hand across the surface without any discomfort. 

 

I don't know if the dense concentration made the difference, or if something else was at play - both pieces were stoneware fired at ^10.  I also don’t know why on my piece some pellets disappear, only leaving a hole where they once were, while others just become a globule in place - it might have to do with original size.

 

Does anyone here have any thoughts on this?

 

[glazenerd]

Charles:

clay and minerals are constantly subject to environment in which they are deposited. The term is morphology. The parent mineral formation, subjected to other minerals in its environment, weather conditions, and time = final material composition. The perlite in the USA can have higher natural fluxes, than perlite in Aussie. The clay you use may have higher flux content than the clay used in Aussie: both effects outcome.

A forum member was kind enough to send me a sample of clay she found locally. The natural conditions produced over 20% flux content: so much so, a white film was visible. Months ago a clay plant manager from India contacted me about clay issues. The acidic conditions of that region had stripped nearly all natural fluxes from the clay. Much depends on environment as well.

t

[Rae Reich]

1) More flux in white clay than in red

2) Red clay was reduction fired, white clay perhaps in oxidation 

3) Variations in perlite composition 

4) Variations in perlite particle size 

5) Perlite appears to have been wedged (or maybe mixed into casting slip) throughout the white clay, not applied only to the surface. Whole form would fire differently if less dense. 

6) Tests with perlite to determine its melting/burnout temperatures would be useful. 

[Charles]

Thanks to both of you. 

I was planning to try wedging tonight, and will do that with porcelain on two test pieces, one for each firing method. 

I had asked the studio manager if the ball clay would make any difference, and he doubted it.

The school has frequent reduction firing, but I'll have to sign up for an electric kiln for oxidation, and will need to wait until I have other pieces to justify that.

I'm also checking with ceramic sources in Australia to see if it might be the Perlite there.

 

[Rae Reich]

You might be able to recruit other studio members to help you fill the kiln. 

Where did you get your perlite? Garden supplies?

Good luck!

[Charles]

Rae -

The studio manager says he can work with me to make a small test batch of ball clay.

I can see how the plasticity of a ball clay would help here.

I realized that using Porcelain for this was not a good idea, as I've had bad luck trying granite inclusions in it - the strength just isn't there.

I tend to doubt that the firing method would really make much - if any, difference in this regard – based on my minimal knowledge.

Yes - garden supplies, if there is any variation just how can one know about it?  I don't know if it would just be in the source rock, or could be introduced in the expansion processing.

Charles

 

[neilestrick]

If you want stuff that actually burns out and leaves voids, I would go with something organic, that is definitely going to burn out- coffee grounds, popcorn, nuts/seeds, etc.

[Rae Reich]

2 hours ago, neilestrick said:

If you want stuff that actually burns out and leaves voids, I would go with something organic, that is definitely going to burn out- coffee grounds, popcorn, nuts/seeds, etc.

Yes, you're right, but I think @Charles wants the shard-y look, like the example, rather than "roundy-er" organics. More like rock salt or crystallized sugar, but are they too water-soluble? Rough-cut celery, onions, zucchini? (You can tell I've been cooking) Possibly crushed walnut shells - can't remember what I heard they are used for - but they wouldn't shrink with the clay. Do they need to shrink with the clay? How did @LeeU's coffee grounds experiments turn out, I wonder? Maybe hammered coffee beans (mild, non-oily roast)?

.....Possibly non-productive musing. ...

[Charles]

1 hour ago, Rae Reich said:

Yes, you're right, but I think @Charles wants the shard-y look, like the example, rater than "roundy-er" organics. More like rock salt or crystallized sugar, but are they too water-soluble? Rough-cut celery, onions, zucchini? (You can tell I've been cooking) Possibly crushed walnut shells - can't remember what I heard they are used for - but they wouldn't shrink with the clay. Do they need to shrink with the clay? How did @LeeU's coffee grounds experiments turn out, I wonder? Maybe hammered coffee beans (mild, non-oily roast)?

.....Possibly non-productive musing. ...

Not necessarily "shardy", but with some texture and variation, as in the originally referenced piece.  It was the texture that I responded to when I first saw the photo, as well as a bit of mystery, with a sense of possible hidden caverns.  I'd be happy to go organic, but Rae  expressed what interests me quite well. 

I've attached a detail image of Louise Gregg's piece that I found on her site.  This makes it seem a very non-dense clay, almost more of a paper clay or similar.

BTW - crushed walnut shells are used as an abrasive blast media (I looked at them online, but they appear quite boring), along with coal slag, which Perry Haas uses quite effectively in his work.  I saw a comment somewhere that this works much better with porcelain, but doesn't do much of anything with stoneware.  After looking for a source to buy a small amount from to experiment, the manager of a local sand blasting company gave me a pound of Black Beauty medium for free.

I've used coffee grounds and they produce a very slight dimple where they were on the surface.  Not at all dramatic, but might serve as background for a bolder texture.  I've also tried Lapsang Souchang tea, which produces a pleasing surface texture, but very quickly grows a white beard when left under plastic ;-)

I also like using mixed small chips of several clay bodies, or grog from them.  Invariably some fall out while I work with the slab, but I consider the recesses part of the texture.  I like using small pieces of granite, which I smash from chicken grit.  In some parts of the country crushed oyster shells are the standard chicken grit, and I've heard of lesser rocks being used - and even labeled as granite, so you need to be careful when shopping.

Two slab-wrap cups with inclusions attached.

 

For those interested, I've just ordered a copy of Additions to Clay Bodies by Kathleen Standen.  This can be had for < $20 from UK sellers with free shipping.

[Magnolia Mud Research]

 

Lucie Rie used rice to make voids in some of her work.  
I have used pine bark, saw dust (fine and chainsaw size), leaves, pine needles, string, and paper spit-balls in exterior slips to create textures.  Literature sources include additives such as plastic chunks such as Styrofoam beads, wads of cotton or wool threads, etc. 

All of these techniques require extra monitoring to insure that the bisque firing is fully oxidizing, and that all the combustibles are combusted prior to the glaze firing.   

An alternative route to texture is to coat the surfaces with coarse clay/stone particles that shrinks differently from the main body surface.  The different shrinkage generates open cracks during both the drying stages and during the fusing stages. 

Other sources of additives are crushed bisque ware, crushed fire brick, crushed construction brick (but NOT concrete/cement bricks), bauxite, ... .  

Globs of earthenware clay embedded in high-fire (cone 10) will produce gaps with oozing glazy melts in surfaces.  

I have added potting soil to decorative slips to change the fired texture.  Crushed shells in slip could also be a route to surface texture if you keep the shells near the surface and washout the residual lime after the firing - if the lime is not removed it will slowly expand from rehydration and can lead to adverse structural consequences.  

The book you reference is worth reading, but you will probably gain more insight by just trying 'stuff' readily available from your current environment.  The main thing to remember is to keep additives that require combustion, or the release of gas from decomposition, near the surface; otherwise you risk significant structural difficulties such as bloating or/and a "loss of containment situation".  If you are using a community kiln, it is wise to keep you "Kiln Master" informed of what you have included into your ware prior to firing  - KM's generally don't like surprises. 

LT