Establishes an infra-red spectral identification key for five natural resins - shellac, sandarac, mastic, copal and rosin - based on their absorbance band positions. These resins can be identified when used in mixtures, such as furniture finishes, at concentrations as low as 1%. Computer methods of deconvolution and special subtraction are used to increase this. The method was validated with reference resins, then applied to finishes sampled from 18th- century furniture.