HERAMIC STUDIO 
123 
AMERICAN GRES 
Charles F. Binns 
HE articles by M. Doat will doubt- 
less have the effect of stimulating 
the studio work of the American 
artist-potter and if the learned 
Frenchman should succeed in in- 
ducing native enthusiasts to devote 
themselves to at least one bi'anch of 
grand feu work he will have de- 
served well of the cenntry. The 
point upon which American crafts- 
men need light is that of the native clays. It is manifestly 
impracticable to import the French claj's and there is no need, 
for clays of every type exist in great abundance in the United 
States. The composition of the paste for gres differs from that 
used for porcelain mainly in the fact that the former is nearly, 
if not quite, a natural clay while the latter is a mixture more or 
less elaborate. The chemical anah\sis, however, of a gr^s 
paste would not greatly differ from that of a porcelain. The 
main point of divergence would be the amount of iron con- 
tained. The porcelain paste contains less than one per cent., 
the gr^s often as much as two per cent, or even more. A 
fragment of pure kaolin will bear an intense heat without 
losing its porosit3^ It is of such a refractory nature that the 
porcelain fire would not suffice to produce translucencj^. It 
is therefore necessary to make an addition of some fusible 
mineral to the paste in order that the desired vitrification may 
be secured. The mineral almost exclusively used for this pur- 
pose is feldspar of which a porcelain paste contains from 15 to 
30 per cent., according to the heat w4iich it is to undergo. A 
natural porcelain paste, white and containing the requisite 
amount of fluxing material, does not exist — except for the 
alleged natural porcelain in Japan — and it must therefore be 
artificially produced. This is done by mixing the proper 
proportions of kaolin, feldspar and ciuartz, having due regard 
to the fire it is proposed to use and to the nature of the result 
required. 
The clays used for gres contain exacth' the ingredients 
which kaolin lacks but they are unsuitable for porcelain because 
they do not become white in the kiln. The manufacture of 
gres belongs then to the catagory of coarse or natural wares 
and it is the more to the credit of the successful artist who 
takes this crude material and fashions it as he will. 
The characteristics of a clay suitable for gres are not hard 
to ascertain but it will be necessary to consult a chemist if 
reliable information is sought. The first point to consider is 
the fluxing content. There are several of the constituents of 
a claj^ which contribute to its power of vitrification. Iron, 
lime, magnesia, potash and soda all occur in clay and each is, 
under .certain conditions, a flux. Kaoliu, feldspar and quartz 
as used in poreclarn contain only potash and soda as fluxing 
ingredients, sometimes only one of these but more often both, 
therefore an endeavor should be made to find, in a clay to be 
used as gres, the amount of fluxing material which shall be 
equivalent to that found in porcelain. This of course pre- 
supposes that the same fire is to be used in each case but as a 
matter of fact the fire undergone by porcelain is usualh^ much 
more severe than that to which gres is subjected. The reason 
for this is in the endurance of the clay. 
The shortest and most accurate way to compare the com- 
position of porcelains and gres is by means of the analysis. In 
a work on Ceramic Technology published in London by the 
writer, certain analysis of typical porcelains are given, among 
which are the following: 
Meissen Limoges Cbinese 
SiUca 58.50 70.20 69.00 
Alumina 35-10 24.00 23.60 
Lime .30 .70 .30 
Magnesia trace .10 .02 
Iron .80 .70 T.20 
Alkahes 5.00 4.30 6.20 
Now, the ([uestion is, "Can any single clay be foiuid in 
Anierica which shall closely appi-oach any one of these coui- 
positions?'" Here are some native fire clays: 
New Jersey' New Jersey Pennsyh^ania. 
vSihca 65.85 65.70 63.43 
Alumina 29.48 28.97 30.23 
Lime 
Magnesia 
Iron .85 1.55 1.38 
Alkalies 3.47 3.75 4.91 
The chemical resemblance between the porcelain pastes 
and the natural clays is quite remarkable but this does not of 
necessity mean similar behavior. One of the most important 
factors in a clay, as governing its point of fusion, is the size of 
the grain. A clay containing coarse sand will, other things 
being equal, be much less fusible than one in which the sand is 
fine. There is also the question of color. It may, in fact, be 
said that the main difference between a porcelain paste and a 
gres clay is the color. Nor is this color wholly dependent upon 
iron as some have supposed. For example, the Chinese paste 
contains more iron than the first clay given and yet it is very 
much whiter. There is no chemical reason, so far as present 
knowledge goes, to account for the subtle changes of color in 
clays, especially in clays which are nearly white. Nothing 
but experiment will determine the point. 
There is, therefore, no difficvdty in procuring native clays 
wdiich are suitable for the manufacture of gres. In fact any of 
the clays which are used in the production of sewer pipesy 
stoneware or low grade fire brick wdll prove suitable. The 
requirements are a slightty sandy grain, a good plasticity and a 
point of vitrification about cone 8. This is rather low for the 
best gres which are fired nearly to porcelain heat. In fact a 
brilliant glaze of the true porcelain type cannot be fused 
much below cone 12. 
Gres clays can be well worked on the wheel, in fact niost 
stoneware claj^s are shaped in this manner to some extent. 
In the preparation of the clay for careful work some pains 
must be taken, for a badly prepared clay will cause serious 
losses. All these clays are liable to contain small nodules of 
either iron carbonate, iron sulphide or lime carbonate. These 
may be as small as the head of a pin but if they lie in the ware 
unnoticed thej- will Ijecome "poppers" after firing or cause 
blisters during the fire. The effect of these "poppers" is to 
blow off small fragments of the potterj^ and, of course, after a 
piece is decorated it may be entirely ruined by such action. 
The remedy is always to make the clay into slip and to strain 
it through as fine a sieve as it will pass. Through 100 mesh is 
best, but some of the sandy clays will not pass this, in that 
case a sieve of 80 meshes to the inch must suffice. If the slip 
be made thin it will pass the more freely. It can then be 
allowed to stand until thick and dried to plasticity in ]ilaster 
molds. The glaze for these clays is about the couqjosition of 
Seger cone 4 of which the formula is : 
K2O .3 I AI2O3 Li()2 
CaO .7 I .5 4.0 
and the mixture: 
Feldspar 42 Flint 27 
Whiting 18 
Kaohn 13 ■ roo 
