276 RERAMIC STUDIO 
action Silver and copper may be used either as oxide, favorable to the production of iridescent reflections, the 
sulphide, carbonate, arsemate, phosphate, chromate or any more so as they have a natural tendency to become irides- 
orgamc salt; however, when these metals are combined with cent by themselves under the influence of atmospheric 
sulphur, the deposits have a red tone which is produced agencies. 1 
only exceptionally in other combinations. Silver chloride firing and reduction 
may be used, but, as it is volatile at a very low temperature, >m,~ ^ • e . A n- j 
tii B «c,iitc «v.+™„Li ui.ii *. 4. * j £ clctLUlc > The firing of metallic deposits requires great care, he- 
me results obtained with it are not constant and it has some 4-U if U" i_ J i. • 1 
a ^; rt « «t™- m ,«-™«„4; • t t. i jii.u«wu«: cause the ochre mixture which determines them must not 
action over surrounding pieces. I have also tried arsenic u ■ x j • ^ ^i -■ i • , 
, , Q+1+ ; m „„ ;„ j-«r * t- ^ t . i be incorporated into the underlying glaze, and on the other 
and antimony m different combmations but without anv i i +v , , ir . ■ j L rd • ,, ,. . , 
results <-"uul t"y hand the meta n lc coat must a( jhere sufficiently not to be 
,«„•*.*.*«„ ~^ ~ ^ «„ rubbed off by contact with hard substances. 
GRINDING OF THE METALLIC MIXTURE nl . ,. . .,-. , ,< ^ . . . . Jf . 
^ Old potters, notwithstanding their empirical methods, 
I have tried to find out what could be the action of fired to the right point, as can be seen from the many speci- 
vmegar which the Moors used in their grinding, especially mens they have left us, on which metallic deposits have re- 
so as this tradition has been faithfully preserved and the sisted the wear of age very well. 
Italians who work in the Golfe Juan and Vallauris potteries Having used a glaze the point of fusion of which is 
are convinced that it is absolutely necessary in order to ob- 99 o° C. (cone 08), I have observed that the point of firing 
tain successful results. I have prepared the Moresque must not exceed 650 C. (cone 020), otherwise the ochre 
formula and divided it into five parts which I have respec- mixture is incorporated into the underlying glaze, 
tively ground with vinegar, gum tragacanth, dextrine, When the metallic mixtures are introduced into the 
essence of turpentine and fucus. x I have applied these glaze (formulas 1a to 6a) the firing is carried first to the 
different mixtures on the same piece of glazed faience and point of fusion of the glaze, cone 09, but the reduction is 
have found that results were identical in every case. There made in the same manner as in the first case. The muffle 
is consequently no reason to use vinegar, which was formerly is left to cool down to red glow, about 500 C, then the 
used for lack of a better product, as ceramic colors are al- reduced firing is given, care being taken to keep the temper- 
ways more easily applied when added to some organic mat- ature as uniform as possible all the time, 
ter in place of water. In practice gum tragacanth and fucus Old potters had realized how important it is to have an 
are the most convenient and should be generally used. even distribution of the reducing gases, and as they could 
The metallic mixture is applied over the glaze to the not obtain such regularity in their primtive kilns, they had 
depth of about one and one-half millimeter, and very varied made a kind of inside muffle or cylindrical box in refactory 
iridescent color effects may be obtained by making, for in- clay, the sides of which were pierced with many holes. This 
stance, a design with a copper mixture and covering the was enclosed in brick masonry' built around it at a distance 
whole piece with a silver mixture, or vice versa ; the design of about fifteen centim. , so that the reducing gases emanating 
will appear in more vigorous tones and these tones may be from the fire mouth at the bottom might penetrate into the 
varied ad infinitum by the superimposition or juxtaposition box through the holes. Piccolpasso has given in his manu- 
of the different mixtures. script many figures illustrating the details of this contriv- 
influence OF THE UNDERLYING glaze ance, and Passeri gives us a rather thorough description of 
The glaze or enamel over which the mixtures for metal- e . r . m &' . . , , „ 
lie deposits are applied plays an important part in the final ; . " ; the vases ' after bem S drawn from thekiln 
result. The underlying glazes which are the most favorable (meaning the second or glaze firing) were touched with the 
to the production of brilliant deposits are those containing red colo f and fired for the t third time in a muffle kiln A 
oxides of copper, cobalt, iron, antimony, nickel and chrome, cestone or large vase m the shape of a basin pierced a l 
especially in presence of tin and lead. The Moors evidently over ™ th } oles > and filled ™ th vases colored wlth ^ red ' 
knew this for they always applied the iridescent mixture was P lac ^ d m ^ he klln - , The finn S lasted S1X hours > not 
over white and blue plumbo-stannif erous enamels. more and was done with broom wood. ' ' 
The glazes which must be used in preference to all Thls tradl ^on has been preserved to our day and mod- 
others are green and turquoise glazes of copper, and the e ™ P ott f s use the made muffle described by Piccolpasso, 
blue ones rich in cobalt. At Golfe Juan and Vallauris the als ° USe broom f ° r fu ^ . J have P roved b f °" " that metal- 
former are generally used. hc de P°"ts can be obtained m any kind of a muffle and 
The finest ruby reds are applied over a green glaze wr * an ^ kmd of fl ? eL / have used ^ m ^ «penments an 
corresponding to • ordinary muffle with a damper at the base of the chimney, 
Feldsoar 7 and m order to have the muffle hermetically closed, I had 
q , ' ' this damper made of cast iron and sliding closely into iron 
Sand of Decize iq grooves. When it is closed, the gases coming from the fire 
Carbonate of potash 6 mouth pass between the brick sides and the muffle proper, 
Carbonate of soda ...'.' " .' '. 5 penetrate, through the evaporation hole of the vault, into 
-n 1 1 1 the firing chamber, then go out through the spy hole, which 
Conner oxide <; ls * e ^ °P en ' except when the reduction is made with gas or 
This mixture is thoroughly blended, then fritted and sugar compounds _ 
j , The Italians, from information which Passeri borrowed 
ground wet. 
Very fine Color effects are Obtained over a turquoise 1. There are at the entrance of the Villa des Dunes, at Cannes, on the Med. 
glaze Which has been Splashed with Spots of a grey Or Celadon iterranean shore, twelve jardinieres in red glaze of chrome, which bore no 
glaze made from nickel Oxide. trace of iridescence when they were placed there, about 1890. They are 
Lead glazes colored red by chrome oxide are also very t0 " da y as iridescent as if they had been submitted to the action of «- 
ducing gases. They were made by a pottery which never manufactured 
1. Fucus crispus is much used by European ceramists who call it lichen. It iridescent ware. 
is a sea weed very common in European waters 2 L. Franchet — Les lustres a reflets metaUiques, Paris, 1896. 
