Oil spots on a metallic background
firing to cone 10 in oxidation
The first firing profile
150 deg F an hour to 250 deg F
400 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
A 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
A one hour hold at 1650 deg F
the second firing protocol
150 deg F an hour to 250 deg F
400 deg F an hour to 2050 deg F
120 deg F an hour to 2240 deg F
60 deg F an hour to 2290 deg F
30 deg F an hour to 2300 deg F with a hold of 20 minutes at 2300 deg F
A 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
A one hour hold at 1650 deg F
discussion of second firing profile
This second firing profile differs in the last 60 deg F before the final temperature which is 10 deg F lower.
The "sharp peak" at the top of the firing curve is smoothed down.
The top temperature is reduced by 10 deg F to compensate for the extra heat work of a slower upramp.
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
The glazes mashiko_ZJ_0, mashiko_ZJ_1, mashiko_ZJ_2 and mashiko_ZJ_3 are first seen here, with information re their composition:
Here we see a first attempt to reproduce one of these glazes, mashiko_ZJ_3 on a plate:
Objectives
1. reproduce an oil spot glaze with a metallic background, on larger pots.
2. find a firing method which gives similar results on thin and thick pots for an oil spot glaze with metallic background.
Oil spot with metallic ground on larger pots
Here are the two glazes mashiko_ZJ_1, and mashiko_ZJ_2 on larger pots with the standard, i.e. first firing protocol.
mashiko_ZJ_1 on a 5 inch diameter bowl
mashiko_ZJ_2 on a 10 inch diameter plate
These glazes in the second firing
mashiko_ZJ_1 on a 5 inch diameter bowl
mashiko_ZJ_1 on a 9 inches diameter plate
mashiko_ZJ_2 on a 5 inch diameter bowl
Commentary
Note: The plates have an abstract design consisting of additional distinct
glazes
applied to the bare bisque independant of the background glaze.
Why would a glaze look different on a thick bottomed plate vs. a thin walled bowl?
A glaze cooks from its skin inward. The wall of the pot beneath the glaze is
the last part to reach temperature as the kiln heats,
till it does, it sucks
heat from the glaze, i.e. its a heat sink. A correction for this phenomena
is
to heat the kiln slower near the final temperature reached to allow
the pot to "keep up" with the glaze.
This slowing near the final temperature
will increase the heat work, and could result in a higher final cone
reached
for the firing. This second firing was slowed progressively
for the last 60
deg F, this was compensated with a 10 deg F lower final temperature for the
firing.