These are printers' inks, available in 30ml jars. These inks are undiluted and can be thinned down using white spirit or other suitable thinner. Solid colour - not diluted.
On opening the jar, you may notice a thin hard film on top of the ink. Since the printers' inks are not diluted with any white spirit, the ink forms this thin film. This is quite normal and inevitable. Due to the (dangerous goods) restrictions imposed by Royal Mail, we are unable to dilute the inks with any white spirit. Peel off the film; then, with a paintbrush and a little white spirit, remove any excess ink from the crust. Once you do this, add a little white spirit to the contents in the jar and you should not get that film again. The inks are meant to be diluted with white spirit for painting as they are too thick to apply otherwise. If you have not used printers' inks before, you may need to experiment a bit with the ink/spirit ratios. Should you have any further questions, please contact us on: firstname.lastname@example.org