Silica and Texture
Increasing the silica in a high Titanium glaze reduces the fluidity of the melt.
Each successive increment of silica to the base glaze alkTi_ZJ_6
gives a less glossy surface texture.
The base glaze alkTi_ZJ_6 is a full gloss, nearly dripping off the pot.
The glaze alkTi_ZJ_0 with the first increase in silica, has a textured full
gloss surface.
The glaze alkTi_ZJ_5 with even more silica, has a waxy matte surface.
The glaze with low silica
It is milky white and glassy, liquid like water, it flowed off of the
pot.
The middle glaze with more silica.
Glaze is glossy, though covered in pinholes.
The glaze with the highest amount of silica.
Glaze is a waxy matte, not a glossy glaze at all, except possibly where it is
thinly applied.
150 deg F an hour to 250 deg F
400 deg F an hour to 1800 deg F
300 deg F an hour to 2050 deg F
120 deg F an hour to 2310 deg F with a hold of 20 minutes at 2310 deg F
300 deg F an hour to 1750 deg F then a half hour hold at 1750 deg F
300 deg F an hour to 1700 deg F then a Three hour hold at 1700 deg F
25 deg F an hour to 1650 deg F then a one hour hold at 1650 deg F
K2O .1
Al2O3 .5
SiO2 2.63
molecular percent Silica 61%
K2O .13
Al2O3 .5
SiO2 3.5
molecular percent Silica 68%
K2O .13
Al2O3 0.51
SiO2 4.24
molecular percent Silica 71%
The bubbles in the bottom of the first glaze are the result of liquid glaze
flowing into the base. In this case, the unpopped bubbles are
The first and last glaze are only slightly crazed, but the second glaze is
strongly so. The second glaze has CaO 0.11,
The solubility of the Titanium in the glaze varies as a result of the
increased silica. This, in turn, modifies the color of the glaze.
The base glaze alkTi_ZJ_6 is high in alkali metals. The bases of all three
variations are close, but not identical.
Increasing silica is not normally thought of as decreasing surface gloss, but
increasing silica increases the viscosity of the melt,
Glaze alkTi_ZJ_6
bowl is ~4 inches in diameter
Glaze alkTi_ZJ_0
bowl is ~4 inches in diameter
Glaze alkTi_ZJ_5
bowl is ~4 inches in diameter
oxidation firing to cone 10 in an electric kiln
Firing profiles
Up Fire profile
Down Fire Profile
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
glaze compositions
Empirical Formula glaze alkTi_ZJ_6 :
Na2O .25
Li20 .25
CaO .13
MgO .27
TiO2 0.18
Empirical Formula Glaze alkTi_ZJ_0
Na2O .22
Li2O .24
CaO .11
MgO .3
TiO2 0.17
Empirical Formula Glaze alkTi_ZJ_5
Na2O .22
Li2O .24
CaO .14
MgO .27
TiO2 0.19
Remarks
the result of
an overly thick glaze layer.
the other two glazes have CaO
0.13 and 0.14 respectively. A line is crossed, and properties change. That
small decrease in Calcium gives
the heavy crazing of a slightly underfired
glaze.
The total alkali metals in alkTi_ZJ_6
equal 0.6 molecular equivalents, but only 0.59 in the other two glazes.
which in this case
decreases the gloss of the glaze. Therefore, I attribute the shifts in surface
texture to the
progressive increase in silica from the first to third glaze.