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Member Since 28 Apr 2013
Offline Last Active May 28 2016 03:19 PM

#107690 Longterm Glaze Issues | Sometimes Runny, Sometimes Breaks The Pots

Posted by PeterH on 28 May 2016 - 09:07 AM

All this seems to be drifting well away from the interests of the original poster, and might be better placed in a separate thread.

IMHO it's very speculative silicate-melt theory. 


>So all you have to do is look at the first ionization energy for each element to determine how quickly it will lose its first orbital atom.

[I assume that is a typo for 'how easily an atom loses its first electron'.]


Absolutely true, but to take an example: the sodium atoms in the nepheline syenite became sodium ions before they

were incorporated into the nepheline syenite crystal (after all it is an ionic crystal). That's probably millions of years ago.


>We call it the melt temp in glaze, but in reality it is when it loses it first atom (donor/ reductant/oxidation).

[I assume that is a typo for 'when the atom loses its first electron'.]


Err, I don't think so. The sodium in the nepheline syenite starts out as Na+ and stays like that through any melting process.

I would suggest that most atoms in a glaze don't change their level of ionisation from that in the raw ingredients.

[There are obviously a few exceptions, mostly involving transition metals.]

  • Min likes this

#107064 Has Anyone Actually Made Pavers?

Posted by PeterH on 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

#106897 Luster

Posted by PeterH on 14 May 2016 - 03:37 PM

I remember reading an article which emphasised the advantage of resist techniques for resinate lustres.

You spend most of your time and effort handling resists, and minimal time actually applying lustre.


... found it, the quote was:

This focus will be on the resist technique which I use so as to limit my exposure to lustre; 90% of

my time with lustre is spent in applying inert resist and only 10% in actually applying the lustre.



#102296 White Glaze

Posted by PeterH on 20 February 2016 - 07:16 AM

This is my first official time at making a glaze from scratch. Its a simple white glaze from a Lucie Rie recipe, fired to ^6 oxidation

I associate Lucie Rie with a time long before ^6 became popular. Are you sure that this glaze is intended for ^6?



Had a quick look and found this posting, suggesting a significantly higher firing temperature.

Also note the possibility of crawling when used on bisc.



Miri, I share your love for Lucie Rie's work. I have one recipe (copied long ago from a UK ceramics book but never used) attributed to her. Here is the text (verbatim with commentary) from the book:

Lucie Rie's White (oxidized, 1,250°C)

58 soda feldspar
14 china clay
10 zinc oxide
10 tin oxide
8 whiting
8 flint

This glaze is the famous glossy white glaze used on Lucie Rie's tableware, often stained brown with manganese and copper carbonate on the rims. It can be tried with less tin - 5 or even 2 percent - but is expensive to make, and inclined to crawl when used on biscuit ware, perhaps because of the high zinc content. Lucie Rie's pots, of course, are once fired, which avoids this problem.

#101784 Paper Clay - I'm New To This...

Posted by PeterH on 10 February 2016 - 05:50 PM

Do be warned, it stinks in a new and fascinating way after about 2 weeks when the fibre begins to rot. Make only as much as you need at a go. People will advise you to add a capful of bleach to prevent this, but it only delays the inevitable.


While Diesel's warning is true, all is not lost. Cannot find the original ref, but here

is my memory of it from a 2009 clayart posting.


Many years ago I read a letter in one of the ceramic mags from a potter who developed a

chronic farmer's lung condition, apparently from mould spores given off from "off" paperclay.

So it's not something you want to have around year-on-year.

His solution was very simple, just drying the paperclay into ingots (prob.in a plaster mould).
He reconstituted the nextday's clay in a bowl with a measured amount of water in it. The great

wicking you get from the paper fibre ensured that it was usable by the next day, maybe sooner.

Regards, Peter


#100564 Mishima: Incised Through Wax Resist Or Scraped?

Posted by PeterH on 24 January 2016 - 06:11 PM

If I'm not mistaken, CAD has an article about a ceramist who incises, bisques, and then inlays.  I couldn't find the article earlier, but I'm 99.9% positive it exists and that it's something from this site.


Rings a bell. The one I remember is:


#99756 What Kind Of Relays Do You Use In Electric Kilns?

Posted by PeterH on 15 January 2016 - 09:59 AM

So I have two lines one at +120v and one at -120v that swap about?

Location Newcastle Upon Tyne. England


neilestrick: All that said, kilns in England may be different.


Single-phase UK is [almost] invariably 240V [ish] neutral and live. Although 2-pole switches are a very good thing,

especially considering how close your hands get to the coils.


PS UK neural should be safe, but mis-wiring, wiring failures and unexpected funnies can prevent this.

  • Min likes this

#99586 Spodumene

Posted by PeterH on 14 January 2016 - 07:47 AM

Passed on in case you have bubble-trouble with a high-spodumene glaze.



Some types of spodumene do contribute to the formation of bubbles in the

glaze slurry. You can wash spodumene before use to alleviate this issue

(mix it well in plenty of hot water, allow to settle overnight, pour off the water

the next day and dry it).


#98933 Cobalt Carbonate - 46%

Posted by PeterH on 06 January 2016 - 07:00 PM

Tyler is right, it may just be a change in presentation.


46% "Cobalt Carbonate" seems to be sold as an animal feed (with various information in the MSDSs).


The 46% seems to be the weight % of cobalt.


This MSDS gives the formula as 2CoCo3.3Co(OH)2.H2O and % Co in the range 45-53%



This is the formula for a hydrated basic cobalt carbonate, and my back-of-the-envelope

maths suggests that should theoretically contain 55% cobalt.

[ (5*59)/(5*59+13*16+7*1+2*12) if you want to check I got the calculation right]



I seem to remember a thread on copper carbonate a long time ago (when the colour

of somebodies supply changed), and we realised that neither the new nor the old

product was the CuCo3 that our chemistry teachers told us about.


... it was  New Copper Carb A Way Different Color and  jrgpots supplied the answer

Trying to remember my geology, Malachite, Cu2Co3(OH)2, is a mint green and

azurite,Cu3(CO3)2(OH)2 is bright blue. Both are forms of copper carbonate.



#98888 Guess The Rock.

Posted by PeterH on 06 January 2016 - 07:51 AM

I'm certain that you would find parts of Brian Sutherland's book "Glazes from natural sources" interesting.



[I picked up a copy in a remaindered bookshop, and have never used it "in anger".]



#98769 Slip Cast Ice Cream Cone?

Posted by PeterH on 04 January 2016 - 10:08 AM

Benzene: You might want to fill the cone with something to prevent against collapse, from the weight of the plaster.


Good point, and you will also want to do this with something fairly dense to stop the cone floating about

in the unset plaster. Damp clay is good, but keep contaminated clay well away from your other plaster. 


Cultures vary, but many UK cones have significant detail in the rim.


If you want to mould this (and the rest of the cone suits a drop-out mould) you may want address this

with a some sort of removable spare. (That needs a picture, ask if you are interested.)


I suggest that you

- make a simple drop-out mould of the main body of the cone

- check that the casting does drop-out!
- then, if you would like to share views on multi-part moulds, re-post (with a picture of the cone)

#98725 Gravity Feed Slip Trailer

Posted by PeterH on 03 January 2016 - 07:48 PM

Pugaboo: Look up Airpen


Thanks, that's the one I was meaning.


http://www.silkpaint...irPen Color.pdf


#98052 Insulating Ceramic Cups?

Posted by PeterH on 26 December 2015 - 07:41 PM

You might also consider trying a lid. Heat loss from a styrofoam cup is certainly reduced:


... figure from http://www.madsci.or...84491.Ph.r.html




#96037 Cut, Spiral Bit Warped In Kiln

Posted by PeterH on 19 November 2015 - 04:10 PM

I'd been wondering about a variant of Chris's idea. Fire fully-cut and unglazed to cone 6

with the pot resting on its rim. Looking at the first picture most of the slumping seems to

be happening near the rim. (At the least you are reducing the spiral by its thickest 1800.)


Then low-fire glaze.  BTW glazing a vitrified pot can present problems, unless you are

using a "painting" glaze full of gums.

#95703 Making Sodium Silicate.

Posted by PeterH on 11 November 2015 - 07:47 PM

Seems like a high water content.  Is that normal?


UK suppliers seem to sell "sodium silicate" in two strength 75 & 1400Tw  (degrees Twaddle).

Personally I found the 140 to be very difficult to handle and measure. Haven't tried the 75,

buying Dispex last time I needed a deflocculant.


IIRC most recipes just say sodium silicate and don't give a strength. I've no idea what strength(s)

are normal in the US.


Of the URLs given in this thread so far:

- the Axner ref gives a trade-name but no strength, although recommending diluting 50:50 with water

   - a MSDS for the brand gives a s.g. of 1.39 [=780Tw?] http://www.muirbeach...um_silicate.pdf

- the Mistral ref is 750Tw

- the pure-water refs don't seem to give a strength

- my APC Pure ref gives a strength of 82-860Tw


So this is the strength I would buy for use as a deflocculant (or dilute a more concentrated form down to).


However I'm delighted you asked the question because I realise it probably isn't the right strength for the original poster.

Who said in a later post: I wanted to experiment with it as a refractory glue.


So I now realise that he probably wants/needs the sodium silicate as strong as he can get it.  I haven't done a full

search but as a guide Bath Potters offer 1400Tw at £3.14/500g & £23.08/5Kg; unfortunately plus P&P (£3.70 & £10).



If you do go the DIY route have a look at the price of the silica-gel cat-litters (may have to: pick indicator crystals out,

and dry in the oven).


Watch the H&S. I have regarded caustic liquids a lot more warily since seeing a note pinned up in a wet-chemistry

lab saying that somebody in another commercial lab had managed to dissolving part of his eyeball with N/10 NaOH

(he recovered). N/10 NaOH is/was the standard titration strength and is pretty weak (0.4%?), so stronger, stickier

and hotter solutions deserve careful handling.