RERAMIC STUDIO 
275 
tion of five to eight percent alumina. Similar glazes may- 
be prepared for gres and porcelain. 
I fired these glazes in an ordinary muffle, provided at 
the base of the chimney with a damper, which closes hermet- 
ically to prevent the oxygen from the air combining with the 
carbon monoxide during the reduction. 
When the temperature at which the glaze is developed 
is reached, I let the fire go out and the muffle cool down to a 
red glow, when I give the reduction or smoking according 
to the method which will be described further. 
With these metallic glazes the variations in color of the 
metallic deposits are much more sensitive than they are 
with the metallic mixtures applied over the glaze according 
to the old process. With glaze No. ia I was able to study 
the changes produced by the longer or shorter time of the 
reducing action, and to understand how in the XVI century 
the golden, yellow and brown tones of the iridescent faiences 
of Diruta were produced. The time of reduction was 
certainly very closely watched by Xanto and Giorgio 
Andreoli. 
Thus glaze No. ia passes through five different stages: 
First stage — Brass metallic tone. 
Second stage — Gold metallic tone. 
Third stage — Yellow brown tone, few metallic reflec- 
tions. 
Fourth stage — Blackish brown tone, few metallic reflec- 
tions. 
Fifth stage. Black tone, no metallic reflections. 
The gold tone of the second stage has such a remarkable 
brilliancy that it may be taken for gold. But when gold 
itself is introduced into those glazes in any form it does not 
seem to have any action whatever. 
In the three last stages the metallic brilliancy disap- 
pears but if the ware is fired again in an oxidizing at- 
mosphere, and then submitted to a new reduction, the 
metallic reflections will reappear either with the brass or 
the gold tone. 
Glaze No. 2a retains the copper appearance with varied 
iridescent reflections caused by the presence of silver oxide. 
Glaze No. 3a shows the intense action of bismuth oxide, 
as it always has a strong blue tone which entirely overcomes 
the colors determined by copper and silver oxides. This 
blue glaze is generally mat with a more or less marked 
mother of pearl effect; when combined with glaze No. ia 
it gives a green metallic deposit of very brilliant color hav- 
ing also nearly always a mother-of-pearl effect. 
Glazes No. 4a, No. 5a and No. 6a give deposits with 
very brilliant and varied iridescent reflections. No 6a is 
generally mat. 
I have also tried these glazes, using in place of metallic 
salts the corresponding minerals, replacing subnitrate of 
bismuth by bismuthine (Bi 2 S 3 ) and the copper compounds 
by chalcosine (Cu 2 S ), covelline (Cu S ), chalcopyrite 
(Cu Fe S 2 ) and philippsite (Cu 2 Fe S ). The deposits were 
good but generally of a darker tone. We know that the 
old potters prepared their copper and silver sulphides them- 
selves, as is shown by the manuscript of Martinez de Frugo. 
Some Italian potters who work in the Golfe Juan and 
Vallauris potteries introduce into their mixtures pulverized 
charcoal or bone black, but this substance does not seem to 
have any particular action, neither has the use of copper 
sulphate instead of copper sulphide. 
The many experiments which I have made convince me 
that free sulphur or sulphur combined with the metals is 
not absolutely necessary (formulas Nos. 1, 2, 3, 4, 5 and ia, 
2a, 3a), neither is ochre (formula No. 6). Cinnabar has no 
Hispano Moresque plates in the Metropolitan Museum of Art, by courtesy of the M 
