The Quest For The Path of Glaze formation
In the firing kiln, as it heats up, what are the steps in the migration of
ground-up rocks to molten glaze?
The result of firing a coating of salt on stoneware at cone 01 is shown
here .
Now I coat a tile with a combination of one material and salt, and fire to a
temperature a bit below the
EPK mixed with NaCl and fired to cone 01 is fragmented, broken apart, with
liquid formed at the edges.
The fired form of the NaCl and G 200 EU is surely modified over its pre-fired
condition, that paper-thin lacey
How is it that NaCl and EPK Kaolin mixed and fired produce some liquid, yet
NaCl and G_200 EU Feldspar do not?
tile is ~2.5 inches in diameter
tile is ~2.5 inches in diameter
150 deg F an hour to 250 deg F
400 deg F an hour to 1864 deg F
120 deg F an hour to 2046 deg F with a hold of 20 minutes at 2040 deg F
K2O 0.01
Al2O3 0.62
SiO2 1.3
molecular percent Silica 44.1%
K2O 0.23
Al2O3 0.37
SiO2 2.26
molecular percent Silica 62.2%
The surface of a tile is coated with a layer of either the mixture of NaCl and
EPK, or NaCl mixed with
Few of the ingredients in a glaze melt at
any temperature attained in a kiln.
melting point of Na2O, as in the above note,
this is cone 01.
Where the 'glaze' coating is sufficiently thin,
one might call the result a glaze, it seems fully melted. This is
clearly
seen in the close-up image of that tile.
remains of what appears to be a bubble is not
like any pre-fired glazed tile. Yet there is no evidence of liquid formation.
Is it perhaps the Iron content of the
EPK Kaolin that perhaps acts as a catalyst for the reaction?
Image of the tile with layer of NaCl and EPK .
Close up Image of the tile
Image of the tile with layer of NaCl and G 200 EU Feldspar .
oxidation firing to cone 01 in an electric kiln
Firing profile
Clay body is an imitation cone 6 Bmix from Georgies Ceramic Supply .
glaze compositions
Empirical Formula na-EPK :
Na2O 0.98
CaO 0.01
Fe2O3 0.01
Empirical Formula na-G_200_EU Feldspar :
Na2O 0.73
CaO 0.03
MgO 0.01
Surface coating
G_200_Feldspar. These mixtures are weighed to
give the empirical formulae shown above. Each is mixed with CMC gum.
The
coating is approximately an eighth of an inch thick in center, tapering to
paper thin at the edge.