Firing Saturated Iron Red Glazes

I show the results of firing a saturated iron red glazes in 6 different firings.

Crystal growth in ceramic glazes is via a process of nucleation and growth. The 'starter' crystals form, then grow.
The temperature of maximum growth is sharp and for most of what grows in glazes unknown.
A long slow downfire provides some growth time at whatever that temperature might be.
I am now experimenting with replacing slow downfires with long soaks or holds at a fixed temperature.

One might replace a drop of 150 deg F, in the form of a slow downramp of 50 deg F an hour
by three half hour holds at the intermediate temperatures, or by 6 quarter hour holds at 6 intermediate temperatures,
and eventually for completeness I'll do that. My experience to date is that replacing that slow down ramp
with a single hold is not likely to produce an equivalent result.

The glaze shown here is a varigated iron red, examined closely with a jewlers loup, or a 3x magnifying glass,
one sees a salt and pepper pattern of many colors, all variants on yellows, oranges and reds.
I conclude that holds above 1800 deg F favor the garnets and maroons, below 1750, oranges, yellows and tans are favored.

Note that this glaze is a dense garnet color in firing #5 which has a 3 hour soak at 1850,
while it is an orangy tan in Firing #6 which has no holds above 1750 deg F.

Several of these pots have leaf print decoration, and one notes the sharp boundary between the edge of the leaf and glaze.
This glaze is viscous, doesn't move much, and will do sharply defined surface decorations.

Below are the firing profiles, and pictures of the glazes in each firing.

Firing #1 With slow downfire in two intervals

A half hour hold at 1850 deg F
slow downfire at 50 deg F an hour in the interval 1850 deg F to 1700 deg F
slow downfire at 25 deg F an hour in the interval 1700 deg F to 1650 deg F
half hour hold at 1650 deg F

Firing #2 With a long hold at 1850 deg F

A three hour hold at 1850 deg F
hour hold at 1650 deg F

Firing #3 With a long hold at 1800 deg F

half hour hold at 1850 deg F
A three hour hold at 1800 deg F
half hour hold at 1650 deg F

Firing #4 With a long hold at 1750 deg F

half hour hold at 1850 deg F
A three hour hold at 1750 deg F
half hour hold at 1650 deg F

Firing #5 With a long hold at 1850 deg F and at 1750 deg F

A three hour hold at 1850 deg F
A three hour hold at 1750 deg F
half hour hold at 1650 deg F

Firing #6 With a long hold at 1700 deg F

half hour hold at 1750 deg F
A three hour hold at 1700 deg F
slow downfire at 25 deg F an hour in the interval 1700 deg F to 1650 deg F
hour hold at 1650 deg F

Clay Body

glaze composition

Alkali metal .7
Li2O .2
Alumina .5
molecular percent silica 64
silica:alumina ratio 6.3
alkali metal:alumina ratio 1.4

The pots are small conical bowls ~3 inches in diameter.

glaze satIron_ZG_7 in firings #1 through #6