In addition to the felt hat containing mercury, two hats from the Museums’ collection were found to contain arsenic: a woman’s plaited straw hat with silk taffeta ribbon band from circa 1885 and a woman’s silk hat with jet beading from around 1890. Both by unknown makers, the hats are each distinguished by prominent decorations of taxidermic fowl. The woman’s straw hat features a wing of brilliantly marbled and speckled feathers at its front, while the silk hat has an entire African starling, with wings outstretched, perched above its front brim.

Like mercury’s role in felt-making, arsenic was a key ingredient in historic taxidermy processes. The use of arsenic was popularized by the French ornithologist, taxidermist, and curator for Muséum national d'histoire naturelle, Louis Dufresne (1752 – 1832). Dufresne used arsenical soap to preserve birds for the Muséum’s collection, which became one of the finest collections of its kind in the world. The technique then migrated to England in the early nineteenth century, and reached its heyday in the Victorian age.

A taxidermic bird specimen would be prepared by skinning the bird, hollowing out its body cavity, and leaving the skull, wing bones and legs intact. The interior would then be treated with an arsenical compound – usually a powder or soap – to preserve the skin. The body cavity was filled with stuffing. Wires would be inserted into the specimen to provide support, and could also be used to attach the specimen to its mount, or in some cases, to a hat.