Fabienne on fri 13 mar 98
Hello,
First, thank you Ron for answering my previous question regarding the
cristobalite inversion.
Next question for the experts :)
Assuming that you have an expansion of 8.66 (10^-6 in/C), which turns up to
be that of my clay, how much lower would the expansion of the glaze have to
be to give me a good _theoretical_ starting point before I go and dip some
tiles in it, and start boiling and freezing them? Any other pointers are
welcome :)
Thank you
--
Fabienne | Yes, I have learned from my
http://www.vivelafrance.org | mistakes...
| I can reproduce them exactly.
Ron Roy on mon 16 mar 98
Hello again Fabienne,
Well it's not as simple as that - of course. The factor you are missing are,
1. Calculated expansion of clay is unreliable because it does not take into
account free crystalline silica in the body. Actual measurement of the body
expansion is necessary.
2. Glazes freeze at different temperatures - In order to ascertain fit by
calculation (or by measurement) you have to use the average body expansion
up to (or down from) 50C below the freezing temperature of the glaze. Just
to make that a little clearer - when glazes are still pyro-plastic (soft)
they can still adjust to the body. It is only a problem when both glaze and
clay are solid (and joined) that the different rates of each can set up the
stresses which can lead to fit problems.
The way glaze calculation works best - is by giving you some idea - as you
adjust the glaze by changing the oxides in the molecular formula - of
whether you are moving in the right direction.
To ascertain the correct approximate expansion rates for glazes involves
actual testing of glazes on each body you use - subjecting them to extremes
of temperatures - freezing and boiling.
There are other factors involved like final temperature the ware is fired
to, the kind of clay used and the different types of glazes used - like
gloss or matte.
To answer directly the question - how much lower has expansion of the glaze
to be to stop any eventual crazing - at stoneware temperatures with a
vitrified body - I cannot answer and there seems to be a difference of
opinion. Some say 10% but I don't know yet what the optimum would be. If
two different glazes are used it becomes more complicated as well. Basing
conclusions on calculated expansion will not be reliable simply because
there are other factors involved that cannot be calculated.
Lots of if, ands and buts - I suppose I just have to say beware of any
generalizations in this area - Testing the glazes on the bodies is probably
the best advice I can give at this point. I can say though - don't think
you can draw conclusions about a glaze and clay combination when you change
either - at least not till you do the testing.
>----------------------------Original message----------------------------
>Hello,
>
>First, thank you Ron for answering my previous question regarding the
>cristobalite inversion.
>
>Next question for the experts :)
>
>Assuming that you have an expansion of 8.66 (10^-6 in/C), which turns up to
>be that of my clay, how much lower would the expansion of the glaze have to
>be to give me a good _theoretical_ starting point before I go and dip some
>tiles in it, and start boiling and freezing them? Any other pointers are
>welcome :)
>
>Thank you
>--
>Fabienne | Yes, I have learned from my
>http://www.vivelafrance.org | mistakes...
> | I can reproduce them exactly.
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
David Hewitt on wed 18 mar 98
I think that this reply from Ron is one of the best balanced views on
glaze expansion figures that I have seen for a long time. I have made
sure that I have copied it for future reference.
The following paragraph, in particular, sums up the use of glaze
expansion calculations very well. Even if you are fortunate enough, like
Ron, to have the facility of measuring both glaze and clay expansions,
testing of glaze on clay in a firing is necessary for final proof.
>The way glaze calculation works best - is by giving you some idea - as you
>adjust the glaze by changing the oxides in the molecular formula - of
>whether you are moving in the right direction.
Please excuse me for taking up space by repeating his reply in full but
it needs to be read in full in order to appreciate all the case
presented. Also you may have missed it the first time.
David
In message , Ron Roy writes
>----------------------------Original message----------------------------
>Hello again Fabienne,
>
>Well it's not as simple as that - of course. The factor you are missing are,
>
>1. Calculated expansion of clay is unreliable because it does not take into
>account free crystalline silica in the body. Actual measurement of the body
>expansion is necessary.
>
>2. Glazes freeze at different temperatures - In order to ascertain fit by
>calculation (or by measurement) you have to use the average body expansion
>up to (or down from) 50C below the freezing temperature of the glaze. Just
>to make that a little clearer - when glazes are still pyro-plastic (soft)
>they can still adjust to the body. It is only a problem when both glaze and
>clay are solid (and joined) that the different rates of each can set up the
>stresses which can lead to fit problems.
>
>The way glaze calculation works best - is by giving you some idea - as you
>adjust the glaze by changing the oxides in the molecular formula - of
>whether you are moving in the right direction.
>
>To ascertain the correct approximate expansion rates for glazes involves
>actual testing of glazes on each body you use - subjecting them to extremes
>of temperatures - freezing and boiling.
>
>There are other factors involved like final temperature the ware is fired
>to, the kind of clay used and the different types of glazes used - like
>gloss or matte.
>
>To answer directly the question - how much lower has expansion of the glaze
>to be to stop any eventual crazing - at stoneware temperatures with a
>vitrified body - I cannot answer and there seems to be a difference of
>opinion. Some say 10% but I don't know yet what the optimum would be. If
>two different glazes are used it becomes more complicated as well. Basing
>conclusions on calculated expansion will not be reliable simply because
>there are other factors involved that cannot be calculated.
>
>Lots of if, ands and buts - I suppose I just have to say beware of any
>generalizations in this area - Testing the glazes on the bodies is probably
>the best advice I can give at this point. I can say though - don't think
>you can draw conclusions about a glaze and clay combination when you change
>either - at least not till you do the testing.
>
>
>>----------------------------Original message----------------------------
>>Hello,
>>
>>First, thank you Ron for answering my previous question regarding the
>>cristobalite inversion.
>>
>>Next question for the experts :)
>>
>>Assuming that you have an expansion of 8.66 (10^-6 in/C), which turns up to
>>be that of my clay, how much lower would the expansion of the glaze have to
>>be to give me a good _theoretical_ starting point before I go and dip some
>>tiles in it, and start boiling and freezing them? Any other pointers are
>>welcome :)
>>
>>Thank you
>>--
>>Fabienne | Yes, I have learned from my
>>http://www.vivelafrance.org | mistakes...
>> | I can reproduce them exactly.
>
>Ron Roy
>93 Pegasus trail
>Scarborough Otario
>Canada M1G 3N8
>Phone: 416-439-2621
>Fax: 416-438-7849
>Web page: Home page http://digitalfire.com/education/people/ronroy.htm
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
Tony Hansen on thu 19 mar 98
I agree with Ron that we cannot nail down a target for how much lower
a calculated expansion should be to fit a body. It is vital to remember
that thermal expansion calculations on bodies are totally unreliable.
Calculation gives you direction. I am fitting a glaze to a body now
and I can't think of a better test than the boiling water ice
water or boiling water freezer test.
--
-------
T o n y H a n s e n thansen@digitalfire.com
Get INSIGHT, Magic of Fire at http://digitalfire.com
Fabienne on thu 19 mar 98
Thank you to all of you who replied on- and off-line. It's an eye opener
and it helps me over a few hurdles that otherwise I would have tripped on.
Everyone seems to agree that it's an inaccurate science, as I interpret it.
However, it's frustrating since when trying to achieve safe and durable
ware, you can never be sure that you have achieved the best fit unless you
have the facility as David points out. Such facility is not cost effective
to set up unless it's your bread and butter from what I gather. Thus, we
are left with rudimentary tests victims of plus or minus errors.
Thank you again
--
Fabienne | Yes, I have learned from my
http://www.vivelafrance.org | mistakes...
| I can reproduce them exactly.
Ron Roy on sun 22 mar 98
Hi David,
Thanks for the lovely comments on my calculated expansion comments.
I have three of your tests done - the glaze and bodiies A and A1 - so far
no big news except the glaze has a higher expansion than the body - this
should mean the #1 glaze tile should have a crazed glaze - and it does not.
Better start testing it - I leave for NCECA tomorrow and will get back at
it in 10 days.
>----------------------------Original message----------------------------
>I think that this reply from Ron is one of the best balanced views on
>glaze expansion figures that I have seen for a long time. I have made
>sure that I have copied it for future reference.
>The following paragraph, in particular, sums up the use of glaze
>expansion calculations very well. Even if you are fortunate enough, like
>Ron, to have the facility of measuring both glaze and clay expansions,
>testing of glaze on clay in a firing is necessary for final proof.
>>The way glaze calculation works best - is by giving you some idea - as you
>>adjust the glaze by changing the oxides in the molecular formula - of
>>whether you are moving in the right direction.
>Please excuse me for taking up space by repeating his reply in full but
>it needs to be read in full in order to appreciate all the case
>presented. Also you may have missed it the first time.
>David
>
>In message , Ron Roy writes
>>----------------------------Original message----------------------------
>>Hello again Fabienne,
>>
>>Well it's not as simple as that - of course. The factor you are missing are,
>>
>>1. Calculated expansion of clay is unreliable because it does not take into
>>account free crystalline silica in the body. Actual measurement of the body
>>expansion is necessary.
>>
>>2. Glazes freeze at different temperatures - In order to ascertain fit by
>>calculation (or by measurement) you have to use the average body expansion
>>up to (or down from) 50C below the freezing temperature of the glaze. Just
>>to make that a little clearer - when glazes are still pyro-plastic (soft)
>>they can still adjust to the body. It is only a problem when both glaze and
>>clay are solid (and joined) that the different rates of each can set up the
>>stresses which can lead to fit problems.
>>
>>The way glaze calculation works best - is by giving you some idea - as you
>>adjust the glaze by changing the oxides in the molecular formula - of
>>whether you are moving in the right direction.
>>
>>To ascertain the correct approximate expansion rates for glazes involves
>>actual testing of glazes on each body you use - subjecting them to extremes
>>of temperatures - freezing and boiling.
>>
>>There are other factors involved like final temperature the ware is fired
>>to, the kind of clay used and the different types of glazes used - like
>>gloss or matte.
>>
>>To answer directly the question - how much lower has expansion of the glaze
>>to be to stop any eventual crazing - at stoneware temperatures with a
>>vitrified body - I cannot answer and there seems to be a difference of
>>opinion. Some say 10% but I don't know yet what the optimum would be. If
>>two different glazes are used it becomes more complicated as well. Basing
>>conclusions on calculated expansion will not be reliable simply because
>>there are other factors involved that cannot be calculated.
>>
>>Lots of if, ands and buts - I suppose I just have to say beware of any
>>generalizations in this area - Testing the glazes on the bodies is probably
>>the best advice I can give at this point. I can say though - don't think
>>you can draw conclusions about a glaze and clay combination when you change
>>either - at least not till you do the testing.
>>
>>
>>>----------------------------Original message----------------------------
>>>Hello,
>>>
>>>First, thank you Ron for answering my previous question regarding the
>>>cristobalite inversion.
>>>
>>>Next question for the experts :)
>>>
>>>Assuming that you have an expansion of 8.66 (10^-6 in/C), which turns up to
>>>be that of my clay, how much lower would the expansion of the glaze have to
>>>be to give me a good _theoretical_ starting point before I go and dip some
>>>tiles in it, and start boiling and freezing them? Any other pointers are
>>>welcome :)
>>>
>>>Thank you
>>>--
>>>Fabienne | Yes, I have learned from my
>>>http://www.vivelafrance.org | mistakes...
>>> | I can reproduce them exactly.
>>
>>Ron Roy
>>93 Pegasus trail
>>Scarborough Otario
>>Canada M1G 3N8
>>Phone: 416-439-2621
>>Fax: 416-438-7849
>>Web page: Home page http://digitalfire.com/education/people/ronroy.htm
>>
>
>--
>David Hewitt
>David Hewitt Pottery ,
>7 Fairfield Road, Caerleon, Newport,
>South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
>Own Web site http://www.dhpot.demon.co.uk
>IMC Web site http://digitalfire.com/education/people/hewitt.htm
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
| |
|
