[1] Nottingham Trent Univ, Sch Sci & Technol, Nottingham NG11 8NS, England
[2] Univ Kent, Sch Phys Sci, Canterbury CT2 7NR, Kent, England
[3] Natl Gallery Art, Dept Sci, London WC2N 5DN, England
[4] British Museum, Dept Conservat Documentat & Sci, London WC1B 3DG, England