Images of copper-migration test with BTA-impregnated Whatman no. 1 paper and Whatman no. 1 control samples without BTA. From the left, untreated Whatman no. 1 paper, Whatman no. 1 paper treated with a co-solvent without BTA, and Whatman no. 1 paper impregnated with BTA. A remarkable reduction of the migration of verdigris horizontally and vertically is observed in the BTA-impregnated paper. The pictures on the right figure were all taken from the verso.