Ron Roy on sun 1 may 11
Lois Sharp has asked me to show her how to calculate a substitution
for GB in a glaze - she has given me permission to do it for ClayArt
as well.
It is necessary to understand what oxides are in the raw materials we
use in order to do this - all the material analysis used here are in
our book except for F4 spar and dolomite so I will include them. If
you need the others I will scan the pages from my workshop book and
send them as a PDF file to anyone who wants them.
Dolomite (a variable material - ask your supplier for your local version)
SiO2 - 1.05
Al2O3 - 0.56
Fe2O3 - 0.40
K2O - 0.07
Na2O - 0.10
CaO - 31.41
MgO - 20.77
LOI - 45.53
Total - 99.87
F4 Spar
SiO2 - 66.77
Al2O3 - 19.59
Fe2O3 - 0.04
K2O - 4.80
Na2O - 6.90
CaO - 1.70
LOI - 0.20
Total - 100.00
I have also used an earlier analysis for GB which is probably closer
to what was used for the original glaze - it is important to note that
there is some MgO in GB - so adding either some dolomite or talc to
replace it is important.
Gerstley Borate 9/97
SiO2 - 11.73
Al2O3 - 1.63
Fe2O3 - 0.48
K2O - 1.38
Na2O - 4.29
CaO - 19.06
MgO - 4.19 <<< note MgO content!
B2O3 - 29.35
LOI - 45.53
Total - 99.79
Lois's glaze from a Carlton Ball book.
F4 SPAR..................... 34.90
F3134....................... 25.40
SILICA...................... 25.30
WHITING..................... 5.60
GERS BORATE 9/97............ 5.20
OM-4........................ 3.60
=3D=3D=3D=3D=3D=3D=3D=3D=3D
100.00
CaO 0.61* *means these oxides included in unity - add to=
1.0
MgO 0.02*
K2O 0.07* KNaO K2O and Na2O added together 0.36
Na2O 0.30*
Fe2O3 0.00
TiO2 0.00
B2O3 0.37
Al2O3 0.27
SiO2 3.69
Si:Al: 13.45
SiB:Al: 14.80
Thermal Expansion: 466.49
Formula Weight: 335.81
My revision.
F4 SPAR..................... 32.00
F3134....................... 32.50
DOLOMITE.................... 1.00
SILICA...................... 24.00
WHITING..................... 4.50
OM-4........................ 6.00
=3D=3D=3D=3D=3D=3D=3D=3D=3D
100.00
CaO 0.61*
MgO 0.02*
K2O 0.06* KNaO =3D .37
Na2O 0.31*
Fe2O3 0.00
TiO2 0.00
B2O3 0.37
Al2O3 0.27
SiO2 3.68
Si:Al: 13.58 -> was 13.45
SiB:Al: 14.96 -> was 14.80
Thermal Expansion: 467.59 -> was 466.49
Formula Weight: 334.55 -> was 335.81
First step - set up calculator with the same recipe side by side and
be able to change one.
2nd step - take the GB out of the one you can change.
3rd step - add more Frit 3134 so that the amount of B2O3 in the
molecular formula matches the amount of B2O3 in the original (0.37 mols)
GB out and frit 3134 added to make up the lost Boron (B2O3)
F4 SPAR..................... 34.90
F3134....................... 32.40
SILICA...................... 25.30
WHITING..................... 5.60
OM-4........................ 3.60
=3D=3D=3D=3D=3D=3D=3D=3D=3D
101.80
CaO 0.62* MgO 0.00* K2O 0.06* < too much KNaO (.38)from frit
Na2O 0.32*
Fe2O3 0.00
TiO2 0.00
B2O3 0.37 Just right
Al2O3 0.26 down a bit (none in frit 3134)
SiO2 3.73 up a bit - from frit
Si:Al: 14.16 More silica less alumina Si:Al goes =
up
SiB:Al: 15.56
Thermal Expansion: 474.50
Formula Weight: 336.61
See post #2 for next steps
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