The Alchemy of
Watercolors On Porcelain
get a deep color from Iron Sulphate one needs a highly concentrated
solution. To make this, we take 100mls of water and add 100gms of
Iron Sulphate. This solution can be applied to bisqued ware, with
or without resist decoration. In Fig.2 I have applied the resist
material shellac in the form of the stripes and brushed several
layers of saturated Iron Sulphate on top. The effect of the resist
is to preserve the white porcelain body from the colorant. This
technique can be used with any of the metal salts. Sometimes there
will be a concentration of color along the edge, where liquid has
collected. After decorating, it is necessary to carefully remove
any residue of the salts from the shellac (with a sponge or moist
tissue), as this will stain the surface underneath in the firing.
It may become necessary to let the piece dry in between applications.
Iron Sulphate can also be combined with other colorants, eg. Cobalt
have been lucky, that in my research with Copper Sulphate I have
been able to achieve deep reds akin in depth of color to traditional
copper red glazes. To achieve this type of color a couple of things
are necessary. First of all a strong solution of Copper Sulphate
is needed. To get this we dissolve 25-30gms of Copper Sulphate in
100 mls of water. This solution is applied in several layers onto
a bisqued piece. After an hour or two, as the piece dries, blue
crystals should be forming on top of the surface. They may literally
grow in front of your eyes. If there seems too little color on the
piece, more Copper Sulphate may be added. These blue crystals give
an indication of what the piece will look like after firing, only
they will have metamorphosed into a deep red! (In contrast, Cobalt
Sulphate crystals are pink and turn blue in the firing.)
The firing cycle is very important. If the piece is oxidised,
an insipid yellow color will result and the chemical also tends
to attack the clay surface. Work decorated with Copper Sulphate
needs to be fired in reduction, as with copper red glazes. The reduction
should be medium to heavy, and most importantly, the kiln should
be fired down in reduction for the first few hundred degrees, ie.
from 1300°C (or whatever the maturing temperature of your porcelain)
to about 1000°C, to avoid reoxidation of the copper. When fired
in this manner, a deep red should result, with possible grey areas,
where the solution was applied more thinly, also the surface stays
smooth. In some cases there may even be areas with a silver appearance.
Of course a gas kiln will be most suited to this type of firing.
Gold Chloride can be purchased from laboratory suppliers. I must
admit, though, that I have never done this. Instead I use gold lustre
which is readily available from potter's suppliers and contains
gold chloride. I get quite a good color response from it, although
this method isn't exact regarding quantities. I thin the lustre
with conventional lustre thinner (4) . The color can be applied
with a brush or nib and is clearly visible in contrast to the near
colorless water solutions. Gold Chloride is a very reliable colorant
in oxidation and reduction and will result in light to deep pinks,
depending on concentration.
This chemical is a bright orange crystal in the raw state and
is fairly toxic. Its saturation point is about 12%, after this,
it just won't dissolve any more. 12 gms are added to 100mls of water.
Because a large amount of this chemical is necessary to get a good
color response, the 12% solution needs to be applied in several
layers. As with Copper Sulphate, crystals -in this case orange-
should grow on the surface whilst drying. If there don't appear
to be enough, more solution can be applied. Potassium Dichromate
will fire to an olive green in reduction. I find the color can be
improved on by a small addition of Cobalt Sulphate, which can be
applied separately or added to the solution itself. Potassium Dichromate
also lends itself well to the use of resist materials.
is the most toxic and one of the most expensive of the metal salts.
Extreme care needs to be taken when handling this chemical. The
remarkable thing about this substance is its capacity to form an
edge of concentrated color in combination with Phosphoric Acid,
which displaces the color and reveals the whiteness underneath.
I do strongly recommend to anyone wanting to experiment with Uranyl
Nitrate and any of the other chemicals written about in this article
to buy a copy of Arne Ase's book, entitled 'Water color on Porcelain',
Norwegian University Press, 1989, also published in the USA. It
has all the information on how to use the metal salts and also what
protective measures should be taken.
WOP and Glazing
Normally I don't glaze my work at all. As it is vitrified it will
hold liquids without any problem. But the main reason is that the
smoothly sanded surface of the unglazed porcelain offers a tactile
experience unlike any glazed pieces. Nonetheless, in the case of
functional ware that is to be used on a daily basis, it is necessary
to glaze the work. In some cases I may glaze only the inside, in
other cases the whole piece. Because any application of moisture
on the bisqued ware will affect the applied metal salts, a different
approach is needed for glazing.
One solution is to fire the decorated work to the maturing temperature
and then glaze at a lower temperature in a subsequent firing. This
requires the work to be carefully heated with a burner and the glaze
to be meticulously sprayed on with a spray gun. Of course it will
be essential to experiment to find a glaze that will fit the porcelain
body and the application will need a bit of practice. I use a wide
firing high gloss glaze and fire to about 1100°C.
even this technique will work with only a couple of the metal salts
without changing their appearance. This is the case with Cobalt
Chloride and Uranyl Nitrate. There may be other colorants this glazing
technique will work with, eg. Bismuth Nitrate or Vanadium Sulphate,
but this needs to be looked into. In any case glazing in this manner
will not work well with Copper Sulphate, Iron Sulphate or Potassium
Dichromate without blurring the crystaline structure of the surface,
and if the glazing is done in oxidation (which I normally do), copper
will tint the glaze green.
Calligraphy & WOP
Because the WOP are chemicals dissolved in water and are much
like ink in consistency, it is possible to use them not only with
brushes, but also in conjunction with nibs and writing pens. Some
of the metal salts will give very reliable results used in this
way. Notably those colorants that need only one application can
be used for writing on clay. Best of all are Gold Chloride, Cobalt
Chloride and Uranyl Nitrate. To stop the colors from migrating,
it helps to add some gum arabicum to the water solutions -this thickens
the liquid slightly and stops them from blurring. Needless to say
that the same precautions must be taken here as with any other decorating
technique using metal salts, and extra care needs to be taken not
to smudge already decorated areas. These problems aside, writing
adds another dimension to the possibilities of the WOP technique.
The technique of using watersoluble metal salts is still fairly
new and offers plenty of scope for experimentation. While no iron
will be magically turned into gold, the WOP technique can yield
some really fantastic results, but it can also be disappointing.
colors will not always turn out as expected. Metal salts can be
fairly unpredictable, because there are so many variables that will
influence the outcome (eg. kiln atmosphere, amount of color, degree
of vitrification, etc.). But this very same aspect of unpredictability
makes it an exciting and sometimes very rewarding technique to work
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