/ smalt /

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About the chemical structure:

Chemical name: potassium glass containing cobalt

SiO2(65%) + K20 (15%) + Al2O3 (5%) + CoO (10%)

Crystal system: glass
Refractive index: 1.46 - 1.55


Color Index (C.I.) PB32

How can you identify Smalt?


OM: If ground too fine, it loses its color, therefore the particles of smalt found in paint layers are usually quite large. 


It's identified by means of FTIR and Raman.

Raman spectra: University College London;

FTIR spectra: IRUG

Usage and handling:

Permanence: Toxicity:


Degradation processes: like all glass based pigments, it is stable unless improperly made, and is better in aqueous media and lime for fresco. Anyway, smalt is known to fade. Smalt discolors in oil, as the cobalt migrates out of the glass into the oil, leaving behind an unsightly olive green color.  It eventually deteriorates to a mottled gray because of reaction of the alkali content of the smalt with the oil medium.  The admixture of lead white prevents discoloration to a degree. 

moderately toxic. Smalt contains cobalt, which can be toxic if inhaled or ingested. Care should be used in handling the dry powder pigment to avoid inhaling the dust. May be harmful if swallowed. May be harmful by breathing dusts or mists.

MSDS: Natural pigments


Binger, H., Das Blaupigment Smalte, Restauro, 1, 1996, p. 36-39
Artists’ Pigments. A Handbook of Their History and Characteristics, Vol. 2: A. Roy (Ed.) Oxford University Press 1993, p. 113-130