Civil Engineering Reference
In-Depth Information
or organic. There are two types of inorganic pigments: earth pigment and min-
eral pigment.
Earth pigment occurs ready-to-use in certain types of earth. It is composed of
the decaying products of particular types of stone, and has good durability.
Extraction occurs during washing of the earth. After it has been collected in a tub,
water is added and the mixture stirred. When all the earth has sunk, the water is
poured off and the uppermost layer of fine earth is treated in the same way. This
is done five or six times. The earth is then ground in a mortar, adding water. It is
finally dried and the binder is added.
Mineral pigment is obtained by cleaning natural minerals. Synthetic mineral
pigments are extracted by burning (zinc white), calcination (ultramarine) or pre-
cipitation in a solution (chrome yellow). Compared with the natural earth
colours, the synthetic variations are purer, which makes it difficult to reconstruct
colours in ancient buildings. All inorganic pigments are made synthetically
nowadays, with the exception of umber.
Organic pigments have less durability and fewer lasting qualities than the
inorganic pigments. Pigments used in modern painting are usually made syn-
thetically. One natural organic pigment is coal black, which is made of charcoal,
preferably from willow, beech and maple. Organic pigments are not normally
used nowadays for painting buildings, with the exception of some blue and
green variations.
Many mineral pigments are based on limited or very limited reserves. The pro-
duction of pigments normally has high energy consumption and pollution rates.
This is particularly the case for cadmium, chrome, manganese and lead products;
pollution occurs in the factory environment and when the waste is deposited in
the surroundings. The production of white pigments also causes a great deal of
pollution, particularly in the case of titanium white. The production of zinc white
is also a polluting process. White pigments of chalk and ground glass, however,
do not cause problems.
Pigments and siccatives (see p. 411) are relatively well bound within paints,
and they are less chemically active. When paint is sprayed, it is finely spread in
the air as small drops and the pigments can be inhaled. Welding of painted
objects, scraping, sanding or removing the paint with hot air can all produce
the same problem. Warmed zinc can create so-called 'zinc frost', a very painful
fever, but soon passes. Pigments containing chrome are strongly oxidizing and
thereby irritating and damaging to the respiratory system. Zinc chromate can
also cause chrome allergy. Chrome, cadmium and lead compounds are,
amongst other things, strongly carcinogenic. Ferric oxides can be considered
relatively harmless.
In buildings, pigments are normally harmless if they are well bound with
the paint and not too exposed to wear and tear. Children have, however, been
poisoned by licking painted surfaces. Alkyl phenoltoxilates are often used in
