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PeterH

Member Since 28 Apr 2013
Offline Last Active Today, 03:31 PM
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Posts I've Made

In Topic: Sand On Clay?

23 May 2016 - 05:19 AM

>"Additions to clay bodies" - Author: Kathleen Standen

 

The posted link didn't work for me, this one does.

http://ceramicartsda...to-clay-bodies/

... notice the free except (you may have to sign in, well worth it for the other freebies on the site).


In Topic: Manganese Dioxide Vs Oxide?

21 May 2016 - 02:42 PM

>Any information/thoughts/musings are appreciated, of course.

 

File under ramblings...

 

Wikipedia helps/hinders with

 

 

Manganese oxide is any of a variety of manganese oxides and hydroxides.[1] This includes:

https://en.wikipedia...Manganese_oxide

 

Personally I've always assumed that Hans Coper used MnO2, although perhaps with little evidence.

Certainly that's what I used when trying to emulate his 'bronze' effects in the 1970s.

 

PS High-manganese mixtures and saturated glazes can produce some truly magnificent effects,  but

went out of favour when serious H&S issues were suspected. [Indeed Hans Coper's neurological problems

were rumoured/suspected to relate to his use of manganese.]

 

Like many H&S issues there are few hard facts. A fairly random google gives this as a starting point.

https://digitalfire....odgett_409.html


In Topic: Longterm Glaze Issues | Sometimes Runny, Sometimes Breaks The Pots

21 May 2016 - 08:42 AM

Hi,  > I applied thick - it breaks the cups, but this time I applied it thin it still broke them and run down to the foot.

 

I can clearly see crackle in the photos, but no breaks. Can you supply a picture of a couple of the broken cups?


In Topic: Newbie Needs Help With Glaze Cone 6 - Can It Really Be This Hard?

20 May 2016 - 03:51 PM

The section under Alberta Slip Rutile Blue Cone 6 at

https://plainsmancla....php?recipes6=1

may be of help. Especially the caption to the first picture.

 

Looks like you've picked a glaze that's fussy about the firing schedule.


In Topic: Has Anyone Actually Made Pavers?

17 May 2016 - 10:37 AM

FYI this was recently posted on Clayart, it seems relevant to sculptures, not certain about pavers.

 

There's excellent information on claybodies for outdoor hard-freeze environments in Val Cushing's handbook. He gets pretty technical in terms of figuring how suitable a particular claybody might be, but the basics make sense. It did surprise me when I found out about this, because initially seemed counterintuitive. You don't want to use a highfire claybody unless it is 0% absorption, which is hard to achieve. The problem with most highfire bodies is that there is still some absorption, and over time the water absorbs, and then in a hard freeze it can't get out and the piece cracks or spalls. That's the same reason it's so dangerous to refire a piece that has been in regular daily use in contact with water. The moisture has impacted into the piece and can't escape in the firing, and the piece can explode with enough force to destroy everything else in the kiln and even the kiln itself. I have seen this happen.

Most of the architectural terracotta decorating the exterior of so many buildings in New England and across the Midwest was fired to low-midrange - around cone 2 or 3. The idea is to have enough porosity to allow the pressure of freezing water to escape, but enough mechanical strength to keep the pressure from fracturing the clay, and the right terracotta body fired to low-midrange does that. Regular lowfire bodies are of course very porous with inadequate mechanical strength, and we've all seen examples of lowfire pots, sculpture, or tiles left outdoors in hard-freeze climates, where the surface starts to spall off and eventually the piece just disintegrates.
- Vince

Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net