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#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?




#112569 Can Mason Stains Be "mixed" Together? Eg Red + Blue = Purple?

Posted by PeterH on 04 September 2016 - 05:13 PM

In case is is of interest: Triaxial Blend: Stains in a Base Glaze


#112537 Can Mason Stains Be "mixed" Together? Eg Red + Blue = Purple?

Posted by PeterH on 04 September 2016 - 07:00 AM

I'm unclear what range of colours you are trying to achieve. You might try asking a similar question on

a china-painting group as they would probably be more familiar with the problems of colour-mixing a

large range of colours "on demand" from a small palette.


Regards, Peter



AFAIK the people trying really hard to mix ceramic colours from a limited set of "primaries" are the

developers of colour printing systems (for producing transfers and even direct printing on clay). My

understanding is that they use a CMYK colour model. AFAIK such pigments are not generally/cheaply



If you are unfamiliar with cyan-magenta-yellow colour mixing, here is an example with oil paints:


Colours good for this sort of mixing are often called names like process-blue, process-red and process yellow.



#112475 New Here, Have Question About Duncan Glaze Shivering?

Posted by PeterH on 03 September 2016 - 04:32 AM

You really can’t sub a feldspar (the minspar you used if I read your post correctly) and bentonite for the deflocculants that were called for. The soda ash with sodium silicate work together to deflocculate the casting slip.


As Min said.


(Also Digitalfire advises Do not put bentonite in the casting version of this body, the casting rate will slow down dramatically.)


The amazing differences in the property of slips with different level of flocculation can be seen in

a couple of John Britt videos:

... as these videos are not aimed at a slip-casting audience I would just take the message that

control of the state of [de]flocculation is important. Then  I would re-read the article Min mentioned.


Here is another article which discusses deflocculating a slip (using a different deflocculant) which

highlights the problems of deflocculating just enough but not too much.


Go with Digitalfire's recipe as a starting point, and get both the density and viscosity in the right ball-park.


Then try glaze-tests again, as there is no guarantee that this will have fixed you problem :(.




If you still have shivering problems, there are [at least] two course of action you might try.


1) Tweaking Digitalfire's recipe, some possibilities are:

1a) Switching to a different ball-clay, or a mixture of ball-clays (I have no suggestions here).

1b) Invert Digitalfire's advice for crazing -- If crazing is a perennial problem, then use more talc

      and less ball clay -- by using more ball-clay and less talc.


2) Start another thread asking  for recipes for a cone 04 - 05 casting slip.




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#112393 Where Can I Find Soda Glass?

Posted by PeterH on 01 September 2016 - 01:32 PM

In the UK a www.ebay.co.uk search for glass frit identifies several suppliers you might consider talking to.


If you give your the thread your location you might get a more focussed reply.

#112367 Flux Formula Limits For Porcelain

Posted by PeterH on 01 September 2016 - 07:20 AM



Do you have a ball-park figure for how much vinegar you are adding? (I'm interesting in working out

the final carbonate:acetate ratio).



Are you familiar with drop moulds, they might minimise surface disturbances when forming (fun to use too).

#110978 What Clay To Use To Make Waterfall For Cat And How To Use Clay?

Posted by PeterH on 04 August 2016 - 09:32 PM

We bought a waterfall to encourage our diabetic cat to drink more water. A couple of lessons learned:


If you are in a hard-water area pre-boil the water or you will spend forever removing "tide-marks".


You will get hairs in the water that will finish up blocking the pumps filter (nasty to clean too). Much better

to enclose the pump in a fine-mesh plastic bag to catch the fur.


But it was a great success, and gave him a few extra months.

#110977 Mixed My A First Batch Of Glaze

Posted by PeterH on 04 August 2016 - 09:25 PM

This makes my preferred specific gravity for this particular glaze 1.5?


Yes, you've got it. ... I admire your choice of example numbers to simplify the arithmetic.


PS You can always cheat and put (140-20)/(100-20) into google, it gives me (140 - 20) / (100 - 20) = 1.5


#110226 Relay / Magnetic Contactor Question

Posted by PeterH on 17 July 2016 - 06:03 PM

A long shot, but a firm called "Kilns and Furnaces" exists in the UK.




Might be worth a query.

#109823 Rare Earth Oxides

Posted by PeterH on 07 July 2016 - 03:04 PM

What kinds of places stock the lanthanides?


As a fellow UK resident, you might try asking http://www.ctmpotterssupplies.co.uk


I managed to get some cerium from them after it ceased to be in their catalogue. IIRC they

are associated with a company that sells more exotic stuff.


.... From their colouring oxides page

We are happy to supply companies with raw materials for other uses and, in fact, our parent

company specializes in metal compounds and rare earths such as cerium oxide, cobalt oxide,

cobalt carbonate, copper oxide, cuprous and cupric, copper carbonate, erbium oxide, iron chromite,

lithium carbonate powder and crystalline, manganese carbonate, manganese dioxide, neodymium

oxide, nickel oxide, praseodymium oxide, tin oxide, vanadium pentoxide etc and so we can usually

offer a good price for small and medium lots of these and many other materials; for larger lots we

will liaise with our parent company for you.


Edit: put quote in italics.

#109563 Ian Currie Test Tiles Forums?

Posted by PeterH on 28 June 2016 - 04:11 PM

Don't know if this is of any interest/help but I remember an old J. Am. Ceramic Soc. article on silicon

carbide reduction. Among other methods they added SiC into a slip which they bisqued before

applying the glaze. Seems that the SiC only decomposed when it was wetted by the molten glaze.


It might be one way of getting uniform SiC action for all the sub-tiles on a full-tile, and using the same

glaze batches on several full-tiles with different SiC content in the slips.

... But harder to use dancing men, etc.

#109525 Mocha Diffusion

Posted by PeterH on 28 June 2016 - 08:33 AM

That was my problem too, the few times I tried.


Just looked up a few utube videos on mocha diffusion, it's a surprise just how fast they seem to work

(assuming that the videos are uncut and in real-time).


Less than 30secs from bucket to completion on this one


Usability seems to stop after 10secs on this one



I'm truly uncertain how deflocculating the slip will effect things "chemically", but given the mildly acidic

nature of most mocha teas I suspect that soda ash will. Also deflocculation means that you have more

"clay" for a given viscosity, which might effect the drying time of the slip.


One of Robin's slips is given in:


Various slip recipes are good, the most important ingredient being a high
percentage of ball clay.  A basic recipe which will fit most bodies and
which can easily be colored with stains or various oxides would be;  
I have taught this process all over the world and you can use any ball clay,
any feldspar, any kaolin and any 200 mesh silica, flint or quartz.  They will
all work. This slip is good on most clay bodies from cone 04 to 12,  in any
atmosphere. The thickness should be like double cream, or room temperature
10 W 30 motor oil. A liquified porcelain clay slip will not usually work well
since a porcelain body usually contains a maximum of 50% plastic clay material,
the remainder being non-plastic fluxes and fillers such as feldspars and silica.




PS if only for for the humour:


Peter Linford on thu 22 jul 99
Hi Martin,
I too used to mix up disgusting fluids till one day washing brushes
discovered that a mix of oxide and washing up liquid works just as well and
the brushes are really easy to clean.
hope this will help
Peter in Wakefield, Yorkshire.



#108147 Weight Difference Between Commercial Plates And Handmade Plates

Posted by PeterH on 04 June 2016 - 01:25 PM

A Digitalfire article provides an interesting footnote to John's comments on 'Modulus of Rupture'.


The MOR of two bodies was measured unglazed and with five versions of a Butter Matte glaze.

porcelain unglazed=9122 glazed from=5597 to=3811

stoneware unglazed=3955 glazed from=8135 to=3930



#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.]

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