wallace myers on sat 4 apr 98
Would one of the glaze experts give me their opinion of the
following glaze as to food safety?
cone 6 ox.
76.5 redart
23.5 dolomite
5.0 manganese dioxide
Thanks , Wallace
John Post on mon 6 apr 98
Hi Wallace,
I took a look at your glaze using Glazchem software. The glaze is too low
in silica (SiO2). A balanced cone 6 glaze has silica in the range of
2.5-3.5. At 2.14 yours is too low. This will make the glaze soft and
easily scratched and it will make the glaze more likely to leach metals. I
personally would not use manganese dioxide in a glaze that I put in contact
with food unless I sent the glaze to a lab and had it tested.
The alumina level in the glaze is ok. The normal range is .2-.35.
The magnesium (MgO) at .41 is out of the normal range of 0-.3. The
dolomite is what contributes the MgO to the glaze.
I revised your glaze to bring it more in line with the limit formula
ranges. This will probably change the way the glaze looks, but sometimes I
get nice new glazes with the changes. I'm still concerned about the
Manganese Dioxide, so if you plan to use this glaze on food containers, get
it tested.
A simple test you can do to test the durability of a glaze is to soak part
of a tile in vinegar for 24 hours. If the part that was in the vinegar
changes appearance it means that glaze is leaching and susceptible to
attack by acids from food. This little test is not as accurate as a lab
test, but it is a good place to start. Ron Roy may have more to add to
this...
++++++++++++++++++++++++++++++
+++ Wallace's Glaze ^6 +++
++++++++++++++++++++++++++++++
Date: 04/05/98
Dolomite 23.5 23.5%
Redart 76.5 76.5%
-------- ------
100 100 %
Manganese dioxide 5 5 %
===========================
Chemical Analysis
===========================
Na2O 0.01 Al2O3 0.32 SiO2 2.14
K2O 0.09 P2O5 0.00
MgO 0.41 TiO2 0.03
CaO 0.34 Fe2O3 0.09
MnO 0.15
Alumina:Silica ratio is 1.00 : 6.65
Neutral:Acid ratio is 1.00 : 7.01
Alkali:Neutral:Acid ratio is 1.00 : 0.32 : 2.25
Expansion coefficient: 67.8 x 10e-7 per degree C
+++++++++++++++++++++++++++++++++++++++++++
+++ JP's revised Wallace's Glaze ^6 +++
+++++++++++++++++++++++++++++++++++++++++++
Date: 04/05/98
Redart 287.2 71.8%
Dolomite 53.6 13.4%
Whiting 24.8 6.2%
Flint 34.4 8.6%
-------- ------
400 100 %
Manganese dioxide 20.8 5.2%
Notes:
The calcium (CaO, whiting) in the glaze is now higher as this is what
replaced the MgO. The expansion of this glaze is low at 66.4, but I
didn't want
to change too many of the properties of this glaze all at once.
Test this one, see how it works and let me know how it turns out. At
that point, we can make some more changes base on your results.
===========================
Chemical Analysis
===========================
Na2O 0.01 Al2O3 0.35 SiO2 2.73
K2O 0.09 P2O5 0.00
MgO 0.30 TiO2 0.03
CaO 0.41 Fe2O3 0.09
MnO 0.18
Alumina:Silica ratio is 1.00 : 7.88
Neutral:Acid ratio is 1.00 : 8.25
Alkali:Neutral:Acid ratio is 1.00 : 0.35 : 2.86
Expansion coefficient: 66.4 x 10e-7 per degree C
>----------------------------Original message----------------------------
>Would one of the glaze experts give me their opinion of the
>following glaze as to food safety?
>
>cone 6 ox.
>76.5 redart
>23.5 dolomite
>5.0 manganese dioxide
>
>Thanks , Wallace
Happy Testing...
John Post
rp1mrvl@moa.net
Sterling Heights, Michigan USA
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