it was difficult to see a black postmark on a black stamp the color was changed to red in 1841. It was a matter of concern that there was a possibility that the cancellation might be removed from the used adhesive stamp. Many experiments were made to produce a black ink which could not be removed. On 21st July 1840 Rowland Hill wrote that one:
'Mr Donovan, a chemist of Dublin had succeeded in removing not only the black but also the red colour of the obliterating stamps.'