John Termeulen on sat 3 oct 98
Hi Ron
You supplied a very useful piece of information regarding expansion
numbers for oxides. You also offered more detailed information if
needed. Here it goes. I have been using Insight for a number of years
and was always puzzled by the fact that the expansion figures supplied
with Insight did not seem to coincide with the expansion figures when
you give the analyses to our fellow Clayarters. For instance, on the
list you gave the expansion figure for Bao as 14.00. Looking in Insight
the figure reads .129. Using Glaze Chem. it states that the expansion is
1.29. I am certainly interested to have this unknown resolved once and
for all. In advance I want to thank you!
John Termeulen
R.R.4, Trenton, Ontario
David Hewitt on mon 5 oct 98
It all depends on whose coefficents of expansion you use and the units
that you use. I would think that the 14.00 for BaO is using English &
Turner and the units are %Wt. Linear/oCx10-8. The .129 figure could be
West & Garow and the units %Wt. Linear/oCx10-6. In Glaze Chem this could
be x10-7. You can also have the same coefficent expansion expressed as
cubic or percent expansion and the figure is then different again.
You might like to look at the IMC site and the article on Calculating
Glaze Expansion if you want to see more detail on the different
Ceramists and their coefficients of expansion.
http://digitalfire.com/education/people/hewitt.htm
In message , John Termeulen writes
>----------------------------Original message----------------------------
>Hi Ron
>You supplied a very useful piece of information regarding expansion
>numbers for oxides. You also offered more detailed information if
>needed. Here it goes. I have been using Insight for a number of years
>and was always puzzled by the fact that the expansion figures supplied
>with Insight did not seem to coincide with the expansion figures when
>you give the analyses to our fellow Clayarters. For instance, on the
>list you gave the expansion figure for Bao as 14.00. Looking in Insight
>the figure reads .129. Using Glaze Chem. it states that the expansion is
>1.29. I am certainly interested to have this unknown resolved once and
>for all. In advance I want to thank you!
>
>John Termeulen
>R.R.4, Trenton, Ontario
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
Ron Roy on mon 5 oct 98
Hi John,
It's a long story but I will do my best to be brief.
In the beginning - Insight calculated expansion in a way which was not the
standard way of expressing expansion - correct me if I have the wrong
impression Tony.
So I was using the set of expansion numbers recommended in an article by
David Hewitt - the English and Turner set - BaO = 14.00 for linear
expansion. Then Insight changed the basic figures to get in line with the
standard - but proportionally they are the same.
I have been using those I started with so much I decided to keep using them
- simple as that - just got used to them.
What we have to remember here is. The expansion of either clay or glaze is
a very small number and so it is expanded for ease of use. Correct me if I
am wrong here someone - BaO is really 14 x10-8 or 0.00000014. Some of the
other sets use x10-6 and x10-7. It's really just a way of expressing a
number conveniently and cuts down on the size of the number the program has
to display.
It is perhaps difficult to believe but it does not matter what the
calculated number is - the only significant part of all this is - it gives
us some idea - much of the time, in which direction we have moved when
reformulating a glaze to cure a fit problem.
For instance - no mater which set you use - the expansion always goes down
when you add silica to a glaze - because silica has one of the lowest
expansion rates when melted in a glaze. Not so in a clay body but that is
another story.
There is another complication that needs to be considered. Because I have a
dilatometer - I now have a much better idea of when a glaze softens. All
glazes soften on heating (freeze on cooling is a better word here) at a
different temperature. If the glaze is still soft it will conform to the
cooling contraction of the clay - until it freezes - become ridgid. From
then on - because the clay and glaze will always contract at different
rates - stresses build up between the now ridgid but bonded materials. If
the glaze contracts more than the clay you get crazing for instance.
When I make comparisons to the real situation I find the numbers I am using
are actually closer (but more inflated) than the standard way of expressing
calculated expansion. Doesn't matter either way - calculated expansion is
simply a way to estimate whether any adjustment you make to a glaze is
going to help a fit problem or make it worse.
I don't mind trying to explain this again if necessary - better still -
there are others on this list who understand this - be my guest.
RR
>----------------------------Original message----------------------------
>Hi Ron
>You supplied a very useful piece of information regarding expansion
>numbers for oxides. You also offered more detailed information if
>needed. Here it goes. I have been using Insight for a number of years
>and was always puzzled by the fact that the expansion figures supplied
>with Insight did not seem to coincide with the expansion figures when
>you give the analyses to our fellow Clayarters. For instance, on the
>list you gave the expansion figure for Bao as 14.00. Looking in Insight
>the figure reads .129. Using Glaze Chem. it states that the expansion is
>1.29. I am certainly interested to have this unknown resolved once and
>for all. In advance I want to thank you!
>
>John Termeulen
>R.R.4, Trenton, Ontario
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Tom Buck on tue 6 oct 98
Ron (Roy):
You are correct, Tony Hansen's first set of expansion coefficients
(COEs) were 100 times too low, as pointed out to Tony by a professor in
Nova Scotia. So Tony revised his numbers in the next Version of Insight
he issued, and still uses those revised numbers.
Now, it is important, as Ron says, to realize that in glaze work
we are not concerned with the "absolute" value of COEs, we are not
scientists probing into the secrets of matter who would need precise COEs
so that their findings can be universally reviewed.
Potters are concerned with glaze fit, glaze "hand", glaze "look",
and these properties are not easily quantified. We need to be in the
ballpark before we test, then test, and see if refinements are needed.
So, imprecise COEs will work for us.
As I have pointed out previously in posts on Clayart, the unit we
are measuring for a COE is Length divided by Length divided by degrees of
Temperature (scientists use the Celsius degree): to go another step: the
formula is (Delta)L/LxC, that is, the increase or decrease in length of
the glaze layer for a given known Length x the Temperature rise or fall in
degrees (plus for expansion; minus for contraction).
You can picture, therefore, the difficulty that an industral
ceramcist faces when she/he tries to measure COEs with a Dilatometer (as
Ron now is doing). Since many variables are involved, including the
calibration of the Dilatometer, it is small wonder that the COE for a
given Oxide will vary from person to person. So, the Literature contains
different reported values for each Oxide we deal with.
Ron has chosen his values from those reported by British
experimenters; Tony Hansen, Richard Burkett, Bob Wicks, and others accept
data from The Anerican Ceramic Society via Bulletins, Reports, etc.
As one can expect, the actual COE unit is very small (since we are
heating and cooling 1100 to 1300 degrees C in most instances and the pot
sure doesn't expand beyond the kiln). Hence, one could see the COE for a
glaze reported as 5.6 times 10 to the minus 6th power, ie, 0.000 005 6
units of length change per degree C. This also could be reported as
56x10-7 or 560x10-8 (as Ron does).
But as Ron notes, if a COE for a glaze was 650 on his scale and
there was every chance the body ran 430, then crazing would be noticed.
And the way to fix the crazing would be to alter the mix to bring it down
to say 480 (+/-). And that is why the COEs are worth knowing; they tell
you which way to go to get a satisfactory durable glaze.
I hope by this small dissertation that more potters will begin to
see the reason why Glaze Calculation programs are worthwhile.
Til later. Tom.
Tom Buck ) tel: 905-389-2339 & snailmail: 373 East
43rd St. Hamilton ON L8T 3E1 Canada (westend Lake Ontario, province of
Ontario, Canada).
John Termeulen on wed 7 oct 98
Ron Roy wrote:
> ----------------------------Original message----------------------------
> Hi John,
>
> There is another complication that needs to be considered. Because I have a
> dilatometer - I now have a much better idea of when a glaze softens. All
> glazes soften on heating (freeze on cooling is a better word here) at a
> different temperature. If the glaze is still soft it will conform to the
> cooling contraction of the clay - until it freezes - become ridgid. From
> then on - because the clay and glaze will always contract at different
> rates - stresses build up between the now ridgid but bonded materials. If
> the glaze contracts more than the clay you get crazing for instance.
>
> >----------------------------Original message----------------------------
> Hi Ron,
>
> First of all let me thank you for the explanation regarding the expansion
> numbers. I was also helped by Tom Buck and Dave Hewitt and in turn by Tony
> Hansen's site, IMC. It will be a while before I am able to digest all
> information. The gist of the issue that any of the systems works as it is a
> matter of comparison between the oxides.
> However looking at this information I have a few more question. Since you are
> using the English and Turner coefficients what do you use as limit formulas
> for the different glaze?
> You mentioned in your e-mail that your are using a dilatometer??. Can you
> explain what this devise is and how it is used and what it looks like?
> With thanks,
>
> John Termeulen
> R.R.4 Trenton, Ontario
> Canada
> >
Ron Roy on thu 8 oct 98
Hi John,
I am not sure I am answering the right question here - if not ask me again.
Expansion is a different issue than durability, forecasting surface look
and estimating when glazes will melt. I use 2 different sets of limit
formula to help me estimate durability, surface and melt. One set comes
with Insight - ask Tony where he got them. The other set I found in two
different books - one by David Green and the other by Cooper & Royal.
The Insight limits are to be used without B2O3 NOT included in unity with
the fluxes, the other set are to be used with B2O3 included in unity with
the fluxes - I find the later to be more useful in dealing with glazes with
more than a little boron in them - especially those to be fired at cone 6.
A dilatometer is - I just looked through the two most recent copies of CM -
the others are at my studio - to see if the Tuckers add had a picture of me
with it - nope - but I do think there might have been in previous issues.
Anyway - a dilatometer is an electrical apparatus for measuring the
expansion of materials as they are heated and printing the results of that
measurement - usually a graph and/or a set of numbers.
Looks like a big box with dials and meters with a couple of small kilns on
top where the clay or glaze bars are heated - in a fused silica holder. A
fused silica push rod holds the sample being tested against the end of the
holder. As the sample is heated - the expansion is transferred - by the
push rod - to the electrical components which in turn sends the data - in
the case of my old (1975) instrument to a plotter.
The reason 99% of potters have never heard of one - never mind seen one -
is the cost. For a new Orton model its around $15,000 Amr - that's like
20,000 Can. They are standard equipment in any ceramics manufacturing
situation.
So how come I can afford one - good question! Mr. Platt - before he left
for the southern part of America advertised them here on ClayArt - So Frank
and I decided to split the cost - got 2 for $2000 Amr. and the plotter to
boot.
It is not as simple as it sounds however - had to drive down to Orton to
have it calibrated. I calibrate every 25 runs by the way - just to make
sure the data is reliable.
We will being seeing the results of some of the experiments I am doing with
it - just finishing a 21 sample run for Peter Sohngen on the effect of
different materials on cristobalite production in cone 10R stoneware
bodies.
I will let everyone know when and where the results will be published.
>----------------------------Original message----------------------------
>> However looking at this information I have a few more question. Since you are
>> using the English and Turner coefficients what do you use as limit formulas
>> for the different glaze?
>> You mentioned in your e-mail that your are using a dilatometer??. Can you
>> explain what this devise is and how it is used and what it looks like?
>> With thanks,
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
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