National Gallery Technical Bulletin Volume 33, 2012 Colourless Powdered Glass as an Additive in Fifteenth- and Sixteenth-Century European Paintings Marika Spring Introduction

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Experimental appendix

Analysis of paint samples was performed using a Carl Zeiss EVO® MA10 variable pressure SEM, fitted with an Oxford Instruments X-Max 80 mm2 EDX detector, using INCA 350 software (standardless ZAF quantification). The parameters used were 25 kV accelerating voltage, beam current 200 pA, chosen to produce an appropriate count rate, and 60s livetime. The optimum working distance for the EDX detector is 8.5 mm. The majority of the samples were carbon-coated and analysed under high vacuum conditions. Some samples were analysed uncoated under variable pressure conditions, to allow them to be examined subsequently using other techniques. Air was used as the chamber gas, at 50 Pa, allowing a compromise between adequate charge compensation and minimisation of beam skirting effects (note 82).

Corning Museum of Glass standards A, B and D were used to assess the accuracy of the results. In general the measured sodium values were a little lower than the reference values (for Corning A an average of 13.9% was measured instead of the certified value of 14.3%), but were quite stable and under the analytical conditions chosen, specifically the low beam current possible with the large area EDX detector, there did not seem to be significant diffusion of this element. For most of the elements the values were within ± 5–10% (relative) of the reference value, with the least accurate being for those elements such as Ti which were present in the smallest quantity.

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