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#119495 Lustre / Reduction Chemistry Question

Posted by PeterH on 31 December 2016 - 08:15 AM



You might like to try this. You will need:

- A metal canister open at one end a little bigger than your pot.

- Two pieces of chipboard big enough to cover the end of the can.

- A brick or two.

- A newspaper about 1/4 inch thick.

- A bucket of water.

- Rubber bands or string.

- Some charcoal.

- A plastic shot glass or similar.

- Some alcohol (UL meths, US ??)


- Place the first piece of chipboard on the ground, with the tin on top. (This insulates the tin a bit.)

- Cover the bottom of the can with charcoal.

- Put the newspaper into the bucket of water to soak.

- When it's getting nearly time to take the pot out of the kiln take the newspaper out of the water.

   Let it drip a bit then wrap it round the chipboard and secure it with the rubber bands or string.

   You should have a nice flat region of paper that will securely seal the lid of the can.

- Fill the shot glass with meths.

- Take the pot out of the kiln, and place it in the tin on top of the charcoal. Keep it away from the sides.

- Rapidly:

  - Pour the meths into the tin. [H&S keep your head well clear, and wear something like a leather glove.]

  - Place the wet newspaper over the tin to fully seal it.

  - Put a brick or two on top to ensure the seal it good.


WAIT until it cools. Then open the tin. You should be surprised how big a vacuum has been generated,

and the tin should have left a clear compression ring in the newspaper.


Hopefully the pot should be reduced.


If it was a copper-matte "glaze" you may have reduced it so far that there is a layer of metallic copper. You

can carefully reheat this in an oven and watch the colours develop. Unfortunately they fade with time.




A tin may be too weak and implode with the vacuum. I used stainless-steel tea-caddies from a charity shop.


You may want to try reducing the degree of reduction. You might try less meths, open sooner, etc.


PS-2 I've explained [sic] my limited understanding of copper-matte glazes in post #5 of



PP-3 Dedicated to the memory of Heath Robinson.




#119384 Raw Glazing At Cone 6-8?

Posted by PeterH on 29 December 2016 - 10:27 AM

>"Glazes Cone 6 1240C" by Michael Bailey

>they are rare and not cheap - £12.99 cover price, cheapest on Amazon £34......


You can save a little by using one of the bookshops indexed by bookfinder, at the moment the cheapest are £25.16 new, £22.26 s/h.



#119231 Raw Glazing At Cone 6-8?

Posted by PeterH on 27 December 2016 - 08:36 AM

You might be interested in Fran Tristram's Single Firing: The Pros and Cons (Ceramics Handbooks).


New and s/h copies can be found at reasonable prices




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#118935 Silica In Glaze And Body To Ensure Good Fit?

Posted by PeterH on 22 December 2016 - 09:58 AM


- Firstly, I'm not the right person to ask.

- Secondly, yes -- in theory -- its as complicated as that.

- Thirdly, I don't think anybody normally does it that way.


Direct  top-of-the-head opinions on your points


1. Need a clay body that vitrifies at my firing temperature.

If you want your work to be really functional, otherwise you're relying on the glaze for waterproofing.

AFAIK you cannot do this with e/w as normally fired.


2. Know the expansion/contraction of the clay?

You wish. AFAIK it even depends on how you've fired it.


3. Know my chosen glaze recipe is formulated correctly for the temperature and melts right and is going to be durable?

Ideally. The glaze programs can help (assuming that its a glassy gaze). Things like under-firing a clear glaze to get a matt

are not a good idea. Durability has  received a lot more attention after Mastering Cone 6 Glazes by John Hesselberth and

Ron Roy came out.


4. Know how a glaze recipe will act on my chosen clay (COE)?

Some of the glaze programs can give you a estimate of the COE (assuming that it is a well-fired glassy glaze). AFAIK

the general idea it to learn what the estimated-COE is for glazes that do successfully fit your body (as you fire it) and

aim to formulate new glazes to something like that estimated-COE.

In practice I suspect that there is a lot of grabbing recipes from somewhere, finding ones that they nearly fit, and line-

blending to a good fit. [Good pragmatics are better than reliance on inadequate theory.]

Remember to rub-in indian ink or something when looking for crazing it shows things up wonderfully, even fresh cracks.


5. Test it to be sure with some sort of extreme temperature/freezing/boiling test and durability test?

If you're selling to the public it sounds like good insurance. For the hobbyist its still sounds like good practice, especially 

if you are using a porous body.


#118922 Silica In Glaze And Body To Ensure Good Fit?

Posted by PeterH on 22 December 2016 - 06:35 AM

Am I right in thinking that it is the percentage of silica (silica dioxide?) in a glaze and clay body that determines the fit? If both glaze and body have same percentage of silica then the fit would be perfect with no crazing or shivering?


Probably isn't that simple?


I'm afraid not. Indeed counter-intuitive even crystalline silicon dioxide (quartz) and fused 

silicon dioxide (a glass) have quite different thermal expansion properties. So its not

just the amount of SiO2 but also its chemical/crystallographic form.


To emphasise the point, fused silica has a low thermal expansion and is the gold-standard

for low-expansion heat-resistant laboratory glassware. On the other hand, quartz:


Quartz has one physical property that is a bane in ceramics: A sudden 0.5% increase in volume change as it is heated up through a narrow 50C window of temperatures centering around 550C (it contracts by the same amount as it is cooled through this temperature). Cristobalite, not to be outdone, does the same thing but at a much lower temperature and more suddenly (0.8% change in 30 degrees C centering around 200C). While many books and references state that cristobalite inversion happens at 220C, an examination of a graph of its expansion vs. temperature (see link below or google images for the term "quartz vs cristobalite inversion") shows that it is alot more complicated than that. Cristobalite begins expanding suddenly right from the start, the rate of increase accelerates to a near vertical line, then drops off to a much slow rate of increase.


Basically a lot of the SiO2 in a body is crystalline silica, and most SiO2 in a glaze is part of

a glassy matrix.  So its properties are quite different in the two situations.


I'm afraid its back to the usual sources of information:



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#118920 Looking For How To Make Handbuilt Teapot

Posted by PeterH on 22 December 2016 - 05:52 AM

It would be a pity to give up so soon. The deep texture seems such an integral part of those

teapots, and Ginger gets it almost effortlessly with her texture mats.


Just checking if the state of the clay was approximately as described by Ginger:


...and you used a former to bend the slab :



Good luck.


#118861 Need Help - Cone 6 Oil Spot Glaze

Posted by PeterH on 21 December 2016 - 02:43 PM

You may find this discussion -- and Norm's comments -- interesting:


#118771 Looking For How To Make Handbuilt Teapot

Posted by PeterH on 20 December 2016 - 03:05 PM


teapots  site:http://gingersteele.blogspot.co.uk

suggests that the full details were never posted to the blog.


However, the basic idea of the construction of the neck seems to be in:



... and quite a lot of relevant detail seems to be given when describing the construction of a cup:



PS I thought the pattern rang a bell, although Ginger's inspiration seems to be from dressmaking

similar templates are sometimes used in pottery.



#118635 Help! Frozen Studio!

Posted by PeterH on 17 December 2016 - 09:45 PM

Minor point, I've read of problems with frozen Darvan.


Googling gives



Freezes at -5°C. Protect from freezing.  
Partial freezing does not affect the
product’s dispersing properties.


Which is rather ambiguous.

#118584 Crystals Forming

Posted by PeterH on 17 December 2016 - 06:32 AM

John Britt's observations on crystals in his glaze buckets


#117616 Technical Requirements Skutt Kiln

Posted by PeterH on 01 December 2016 - 01:12 PM

Page 10 of this document might be relevant



Might be the kiln sitter manual at



I would wait for Neil's word on it, but I suspect that the sitter can switch more power than the kiln takes.

#117326 How To Create Thicken Slip For High-Relief Surface Decoration?

Posted by PeterH on 27 November 2016 - 06:46 PM

Eye candy:



#116016 Local Clay Test

Posted by PeterH on 10 November 2016 - 07:55 AM

This might be of relevance.

Sedimentation Test

http://www.gaiacolle...exture Test.pdf


Regards, Peter


Added: Remembering the rather squat jar in the illustration, I was going to add that I would use a slimmer one.

Rereading the pdf, they say just that: A tall slender jar is best.

#113316 Does Magnesium Make Clay Gray? Talc Testing

Posted by PeterH on 17 September 2016 - 05:25 AM

... as I was saying.


I cannot think of any obvious yellow pigment that might be involved, although I do feel that

iron might well be involved somewhere.


Looking at the differences between bar 4 and bar 5.

MgO down 1.-7 -> 0.55, Fe stable 0.17 -> 0.16, TiO2 up 0.03 -> 1.06

Which sort of argues for Ti involvement. 

Well TiFe2O5 (pseudobrookite) is certainly yellow enough, as seen in "marbled terra sigillata"


... I would love to hear of any other slip or glaze that uses pseudobrookite as a colourant, and

the conditions under which it can be reliably produced.


It would be nice if the colourant was an Mg compound. Magnesium ferrite (MgFe2O4) is a potential

candidate, but I've usually seen it mentioned as a reddish brown colour:

- as an oil colour


- as an iron-red glaze



Although the colour is described as "presenting a yellow-orange color" here:



Anybody have any thoughts on candidate yellow pigments, and why it only appears in one of the test bars?



Added: This paper suggests that getting pseudobrookite to work as a yellow ceramic pigment isn't that easy,

with a tendency to produce browns.




#112629 Glaze Sintering

Posted by PeterH on 06 September 2016 - 05:25 AM

Not intended as an answer, just adding to the discussion.


A vague memory of the term Tamman temperature lead me to this:

minimum cone for sintering cone 10 glazes



... which implies the idea of sintering may -- for some glazes at least -- not be the really great idea it sounds.

I have found that there is considerable difference in the way
different glazes act to sintering, Some harden up and stick well at
cone 04. Others want to fall off in sheets. Shinos did not work at
all and I decided to put them on at the kiln site.

I have had the same experience. I don't think 04 will
help. A sintered glaze can be more fragile than an unsintered
CMC is probably your best bet (maybe too late now.)


I does make a suggestion which you might prefer to adding gums to the glaze:

Subject got me wondering - could one spray the surface with a cheap fabric
spray starch to keep from damaging the glazed surface?


Regards, Peter


PS Do you have any bentonite in your glaze?