172 
IlERAMIC STUDIO 
GRAND FEU CERAMICS 
VIT.— Glazing 
Taxilc Doai 
HE glaze is a vitrifialole covering 
which is deposited on a ceramic 
piece to' give it the glossy appear- 
ance, which decorates it so harmon- 
iously and is a joy to the eye as well 
as a pleasure to the touch. It 
must essential^ harmonize with the 
material which it covers and be 
vitrified at the same temperature. 
If it covers a common claj^ like 
the faience clays, which vitrify at a comparatively low point, 
its function is mainl3^ that of protection. Such a piece keeps 
its porosity, remains sensitive to the action of water, conse- 
quently also of frost, and is destructible. It is only when 
covered in every part with the vitreous glaze and kept in a place 
w^here it will be protected from atmospheric variations that it 
will reach a certain age. 
If, on the contrarj^, the clay is vitrified at such tempera- 
tures that after firing, it will be proof against the action of water 
and frost, the glaze has its logical function. It is an ornament. 
In that case it may indifferently cover part or the whole of the 
piece without any risk for the latter, w^hich is eternal. This 
is so with gres and porcelain, and for this reason they are the 
ceramic materials par excellence. 
Take a faience and examine it. It is glazed all over, for 
the least point which wrould be uncovered would be the open 
door for the destructive humidity. Consider porcelains and 
gres in their splendid variet3'', whether Oriental or Occidental, 
the glaze is used according to the fancy of the artist and the 
bottom is always uncovered, because the impermeability of the 
body has given it the right to longevity. All the secret of the 
superiority of these two products is there. 
Though faiences can be covered with lead and tin glazes, 
gres and porcelain require a feldspathic and calcareovis cover- 
ing. But whatever its nature, the ordinary glaze, improperly 
called white, must be colorless, brilliant, translucent and 
limpid. It must spread uniformly over the parts which it 
covers without producing blisters or crackles. Its fusibility 
and its dilatation must agree with the firing temperature of the 
body. Too much and too little fusibility are the sources of a 
number of accidents injurious to the piece, not very marked 
perhaps when it is without decoration, but most unfortunate 
if the piece is richly ornamented. Crazing is the most common 
of these accidents with the French porcelain which is very rich 
in alumina. It is caused by a greater shrinkage of the glaze 
than of the body, or vice versa of the body than of the glaze. 
Crackles occur during the cooling off and are announced during 
the opening of the kiln by a sharp and metallic cracking 
noise. It is then of the greatest importance that there be 
harmony between the body and the glaze, and they are 
hardened or softened, according to cases, by reducing or in- 
creasing the fusible element: potash, soda and lime for the glaze 
and for the bod3^ the plastic element, clay. 
There is no rule, no scientific basis on which to estal^lish 
this relation, this same coefficient of expansion of both matters. 
The only guides are experience and trials. The action of the 
fire itself may be the cause of crackles. I use three glazes with 
a basis of copper, this very sensitive metal, which crease 
outrageoush^ in a reducing fire and acquire all their limpidity 
and ricluiess in an oxidising atmosphere. And it will be the 
reverse with others. 
These accidents and their known causes have induced the 
Orientals to produce the intended creasing, called "truite," 
with its close net of crackles which so happily decorates some 
of their products. 
Beginners will do 'well to do as I did, to adopt the glaze 
which is sold with the bodj^ they buy, and which fits it. Both 
are determined b3r series of experiments which it is the dut3' of 
merchants of clays to make. The3^ have all interest to constant 
ly control their product which may vary according to the purit3^ 
of the quarry veins. By bri3'ing a prepared glaze artist potters 
will avoid much unnecessarj^ trouble. 
Having adopted for my own production the hard silicious 
Sevres porcelain called P N (porcelaine noixvelle), and wishing 
to have a glaze which would fit both porcelain and gres, I use 
the glaze of soft type, .called calcare.ous^ which has long been 
studied at Sevres for this purpose. Its composition is: 
Fontainebleau sand 43 
Bougival chalk (craie) 33 
P N biscuit 24 
It is the glaze which is furnished b3^ Mr. Frugier, but the 
sand can be bought from the factor3' of Creil and Montereau, 
the chalk from Loulenc Freres, 92 rue Vieille du Temple, Paris, 
and the biscuit can be obtained bj^ grinding and porphyrising 
unglazed fragments of P N bod\'. This source of suppl3' makes 
it unnecessai-y to speak of the washing, grinding, screening and 
the general preparation of this glaze, which requires a special 
outfit. 
There are five different wa3^s to put the glaze on gres and 
porcelains: Immersion, insuffiation, imbibition with sponge, 
painting with brush, and blending mixed with fat oil. 
I — If the piece is in the condition of biscuit, that is, fired 
but unglazed, like a Wedgwood, the powdered glaze can be fixed 
on its smooth surface, hard like flint, only b3^ using a fat and 
sticky medium. This medium is the fat oil of turpentine, 
which is mixed with the glaze on a rough glass with a palette 
knife. The mixture is blended with a Inrush on the vase in 
successive coats, each being dul\' dried out on a stove or near 
a fire. 
If the piece has been fired and glazed Init it is found neces- 
sar3- to fire it again, either for additional decoration or to re- 
move the flaws of the first firing, the reglazing should be done 
as before, but of course with thinner coats according to the 
amount of glaze which is already on. 
2 — If the piece is baked, that is, if it has alread3' such 
solidit3" and porosity as have been given to it b3' a beginning 
of firing, it can be glazed b3^ any of the processes mentioned 
before, except the fat oil. 
Glazing by immersion is done b3^ dipping the baked piece 
in a bath, which is constantl3^ stirred in ever3^ wa3^ so that the 
glaze will be held in supension in the water and that the heav3^ 
parts will not precipitate to the bottom, as they naturall3'^ tend 
to do. To malce easier this suspension in water of all the 
matters which constitute the glaze, some kitchen salt, or better, 
vinegar, should be thrown in the bath, in the proportion of one 
measure of vinegar for 8 to 10 measures of glaze. Vinegar has 
over salt the advantage of provoking a fermentation of the 
glaze which makes it easier to use. 
The dipped piece being porous, absorbs the water and the 
glaze is deposited on its surface. The qthck or slow passage 
through the bath will determine at will the thickness of the 
glaze. If the piece is thoroughl^^ baked, four seconds are 
sufficient for a good dipping. All the uneven or missed spots 
due to the touch of the fingers or to the running of glaze should 
be verj- carefulty retouched with a sharp blade and a brush. 
If salt is used, it is important to avoid~mineral salt which con- 
tains iron. 
The action of both salt and vinegar is to hold in suspension 
