David Hewitt on sun 28 feb 99
The following is a repeat of a mailing sent two days ago and which does
not seem to have appeared in the digest. As we are most interested to
know other potters experience on this point we are repeating the
mailing. Please excuse us if it appears twice.
Mike Bailey and I have recently carried out some tests which involved
adding colouring oxides to a base glaze recipe which crazed,
particularly on a porcelain clay body. From the results it was clear
that there was a reduction in crazing with additions of Red Iron Oxide
(4%, 8%, 12%, 16%) and Copper Oxide (2%, 4%, 6%), less so with Cobalt
Oxide, but in this last case the amounts of addition were very small
(1/4%, 1/2%, 3/4% and 1%)
The base glaze recipe was:-
Soda Feldspar 450
China clay 50
Bentonite 20
Lithium Carbonate 20
Whiting 370
Zinc Oxide 50
Flint 240
-----
1200
Fired to cone 6 oxidation
Analysis:-
K2O .03 Al2O3 .19 SiO2 1.78
Na2O .10 Fe2O3 .00 TiO2 .00
CaO .70
MgO .01
Li2O .05
ZnO .11
Coeff. of Expansion 6.34 x10-6/oC Linear English & Turner
This reduction in crazing, however, conflicted with the increase in
calculated expansion with the addition of the colouring oxides. This
calculation of course depended on the figures used for these colouring
oxides. English & Turner did not produce coefficients for Fe2O3, CuO and
CoO so I have been using figures gleaned from elsewhere and I am all too
aware of the problem of mixing such sets of figures. If our experience,
as described above, is exactly what others have experienced, then this
would support the conclusion that the coefficients that I have been
using are wrong.
Using Insight I get a similar increase in expansion. However, using
Appen, who is the only Ceramists to have produced figures for colouring
oxides and the other oxides in this recipe, I do get a reduction in
calculated expansion with adding the colouring oxides.
We would be pleased to know what others have experienced. Is it a well
established experience that adding such colouring oxides reduces
crazing?
--
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
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