It was usual to set the traps in the evening, and after waiting a night, or sometimes a night and a day, to draw them up to the surface, when they were generally found to be full of the coveted shell-fish.[815] There were two ways in which the dye was obtained from the molluscs.
Sometimes a hole was broken in the side of the shell, and the fish taken out entire.[816] The _sac_ containing the colouring matter, which is a sort of vein, beginning at the head of the animal, and following the tortuous line of the body as it twists through the spiral shell,[817] was then carefully extracted, either while the mollusc was still alive, or as soon as possible after death, as otherwise the quality of the dye was impaired.

This plan was pursued more especially with the larger species of _Purpurae_, where the _sac_ attained a certain size; while with a smaller kinds a different method was followed.

In their case no attempt was made to extract the _sac_, but the entire fish was crushed, together with its shell, and after salt had been added in the proportion of twenty ounces to a hundred pounds of the pulp, three days were allowed for maceration; heat was then applied, and when, by repeated skimming, the coarse particles had been removed, the dye was left in a liquid state at the bottom.

It was necessary that the vessel in which this final process took place should be of lead, and not of bronze or iron, since those metals gave the dye a disagreeable tinge.[818] The colouring matter contained in the _sac_ of the _Purpurae_ is a liquid of a creamy consistency, and of a yellowish-white hue.

On extraction, it is at first decidedly yellow; then after a little time it becomes green; and, finally, it settles into some shade of violet or purple.