After the silver Daguerreotype plate was polished it was sensitised by exposure to iodine and then, usually, bromine vapours. This was done either separately in two sensitising boxes or in a combined box which had the advantage that the plate could be moved quickly from the iodine to the bromine compartments.

The double box shown here has two thick glass containers (or pots) to hold iodine crystals and bromine water. The polished plate would have been placed face down on a wooden frame and exposed to the iodine vapour and then to the bromine, the frame fitted into a groove and could be slid within the box. The frame would have been able to take plates of different sizes. There are two pull-down flaps on the side of the box, these were fitted with mirrors so that the effect of the sensitising could be judged, on the other side of the box there are two slides that were opened to admit light. The tops of the containers are ground, when not in use they were covered by a sheet of glass. This box is fitted with two thick pieces of glass to cover the containers and these slide within a groove, they are polished and not ground as might be expected and are probably replacements.

A similar box is shown in the Knight catalogues of 1851 and 1853, the difference being that the catalogue shows a box with side pieces to the lid and has springs underneath the containers so that they are in contact with the glass covers, that woodcut is in other catalogues and corresponds to a sensitising box by Horne & Co. in the Science Museum collection. Hogg, Practical Manual of Photography 1853, shows the same box and notes that it from Knight.

The box shown here is probably an earlier example, a drawing in a book by Charles Chevalier dated 1847 shows a box that is closer to this example although it is fitted with levelling screws. Another similar box is shown in a catalogue from Bland (1859). The Bland catalogue also shows a double box described as a 'French pattern' with two draws holding the glass covers to the containers and does not have mirrors, a box of this type is shown in Coe, Cameras.