Lead chromate

Lead Chromate

Color Index Pigment Yellow 34 CI 77600 ; CI 77603

Formula PbCr04 / PbS04

Structure

Lead Chromate pigments can be pure lead chromate (dark yellow pigments) or mixed phase lead chromate and lead sulphate (bright yellow pigments). The most important lead chromate pigments are:

Primrose chrome- a pale, greenish yellow (containing between 45% and 55% lead sulphate), where the pigment has a metastable orthorhombic crystal structure, that requires stabilization by the addition of additives.
Lemon chrome - also a greenish yellow, but redder than primrose, according to the sulphate content (it contains between 20% and 40% lead sul-phate - the more sulphate, the greener the shade). It has a monoclinic crystal structure.
Middle chrome - a reddish yellow shade, typically a pure lead chromate precipitated in the monoclinic structure.
Orange chrome - chemically this pigment is a basic lead chromate and contains no sulphate. It has a tetragonal crystal structure.

Properties

Yellow Lead Chromates have very bright shades and high chroma, making them ideal for full shade yellow paints.

Properties of lead chromate
Property	Lead Chromate
Opacity	excellent
Oil absorption	low
Solvent Resistance	excellent
Heat Stability	200°C

Problems

Color
Fading can occur because they are sensitive to alkalis and acids. Their light fastness is usually satisfactory in full shade, but they darken on exposure to light.

Toxicity
Lead chromes contain both lead and chromium (VI). This effectively limits their application to certain industrial finishes. Warning labels are required on european paints using these pigments. Disposal of waste can also be problematic and local regulations carefully followed.

Surface Treatment

Lead Chromates can be surface-treated with silica, antimony, alumina, or various metals. This surface-treatment permit to improve their properties and to reduce their problems.

Price

All pigments are reasonably cheap to use and replacing them cannot be done without a large increase in cost.