Beth Compton on sat 30 oct 99
Can anyone give me the formula to figure out the coefficient of
expansion ( hope I spelled that right) for any clay body and any glaze?
Tom Buck on sun 31 oct 99
You ask for help on Coefficients of Expansion/Contraction.
Welcome to the world of emprical science/technology. At first glance,
contraction/expansion seems easy.... the maths looks so:
Coefficient of reversible linear thermal expansion = delta-L
divided by L x oC and hence has the "dimension" of 1/oC (or oC-1).
("delta-L" means the tiny change in length per degree C).
Once the glaze goes solid (below 1000 oC) the glaze contracts fairly
uniformly to room temperature, and the total contraction is Coefficient x
degrees C (ie, the change in temperature).
Each Oxide component of a glaze contributes its share of
contraction/expansion, and the combination of all Oxides results in a sum
of individual contributions, and this yields the CoE ("CTE"). The Sum of
such a Combination arises from the number of molecules (moles) of each
Oxide and the CoE that such Oxide exhibits (a value found experimentally).
It would be most tedious to calculate by hand such a Sum of
individual CoEs, especially since the various computer programs do it
The Literature cites CoE values for the various Oxides used in
The CoE of a claybody must be determined experimentally; it cannot
be be calculated with any reaonable accuracy because 1) Most particles do
not melt; and 2) Some clayey materials undergo restructuring (which is
generally uncertain). So any assumption of uniform behaviour simply is
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
David Hewitt on sun 31 oct 99
May I suggest that you look up an article that Mike Bailey and I wrote
on 'Calculating Crazing' and which can be accessed from either of the
web sites listed below. This should give you a start to the subject.
In message , Beth Compton writes
>Can anyone give me the formula to figure out the coefficient of
>expansion ( hope I spelled that right) for any clay body and any glaze?
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ, 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