Important evidence from archaeological sites is often not visible to those excavating because it is so small, or barely distinguishable from the sediment in which it is embedded. This is particularly true for Stone Age sites, as the process of manufacturing stone tools creates tiny fragments that are quite difficult to identify whilst trowelling. Although small, this material is important to our understanding of the manufacturing processes that led to the creation of specific tools. Other types of evidence that are not readily apparent are fragments of charred hazelnut, seed, charcoal, and even bone. Because of the rarity of the Mesolithic structures such as that found at Howick, it was decided that all the sediment which was trowelled through during excavation was collected by context in bags and subjected to flotation.

The flotation process consists of filling a tank with water, and allowing the water to continue flowing so that there is a constant overspill. A mesh is suspended slightly below the waterline, and the sediment bags are emptied on to this. The heavy sediment, such as the sand grains, falls through this to the bottom, but objects larger than the mesh are caught in it. Lighter objects, such as seed, bone and charcoal nevertheless float to the surface. This material is carried on the water that is overspilling into a further series of fine meshes, and ultimately caught.

The material collected from all the meshes used is then dried, and bagged according to the context from which it was collected. This is then sent off to respective specialists in the analysis of flint, seed, bone, hazelnut, etc.

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