Flotation machines

An  archaeobotanical flotation photo gallery

This is typical froth flotation machine, often constructed from an old oil drum. It is also known as a Siraf-type flotation machine.

Flotation machine at Giza, Dr. Mary Anne Murray hard at work.

Although they come in various guises, flotation machines involve water that is pumped in from below the sample, mixing it and agitating it, and the flot that spills over a spout into the collecting sieve (or series of nested sieves).

The simple animation here (opens in a new window) (courtesy of Ksenija Borojevic) illustrates the principle. For a how-to slide show on machine flotation, see this one put together by UCL student Leilani Lucas.

The picture at left shows a sample being added to a flotation machine, and the overflowing water (with charred finds) being caught by pouring through a pair of larger to smaller sieves. (The Photo is by Mark Nesbitt at Halam Cemi (Turkey) and is taken from Wikipedia.) For an annotated diagram of a flotation machine of this kind, download this PDF (2MB) 

 Within the flotation tank a metal screen holds the weight of the sample and separated from the water spout. Beneath the sample another mesh usually placed (e.g. of 1mm or 2mm) which acts as a wet sieve to retain bones and artefacts (and stones). Fine sands, silts and clays settle out into a sludge at the bottom of the flotation tank beneath the water spout, and need to be cleaned out periodically. Flotation machines require a reliable energy source (usually petrol) to run a water pump of sufficient power (such as a 2 or 3- horsepower pump run through a 4-8 cm pipe). Machines need to be purpose built, welded together and equipped with appropriate pipes, hoses and mesh. Old oil drums are a popular basis from which to construct flotation machines. They can be cumbersome to move and are therefore most often built at long-term multi-season excavations. Some flotation systems are set up with additional settling tanks , where clays settle out of used water, and water is then reused through the system in order to conserve local water supply.

 
For some literature on this kind of flotation machine, see:

French, D. (1971) An experiment in water-sieving. Anatolian Studies 21: 59-64

 

Jarman, H. N., A. J. Legge and J. A. Charles (1972) Retrieval of plant remains from archaeological sites by froth flotation. In E. Higgs (ed.) Papers in economic prehistory, pp. 39-48. Cambridge: Cambridge University Press

 

Williams, D. 1973. Flotation at Siraf, Antiquity47: 288-292

 

Stewart, R. B. and W. Robertson, IV (1973) Application of flotation technique in arid areas. Economic Botany 27: 114-116

 

Limp, W. F. (1974) Water separation and flotation processes. Journal of Field Archaeology 2: 119-123 [jstor]

 

Watson, P. J. 1976. In pursuit of prehistoric subsistence: a comparative account of some contemporary flotation techniques. Mid-Continental Journal of Archaeology 1(1): 77-100

 

Keeley, H. C. M. (1978) The cost-effectiveness of certain methods of recovering macroscopic organic remains from archaeological deposits. Journal of Archaeological Science 5: 179-185

 

Nesbitt, M. 1995. "Recovery of archaeological plant remains at Kaman-Kalehöyük," in Essays on ancient Anatolia and its surrounding civilizations, vol. 8, Bulletin of the Middle East Culture Centre in Japan. Edited by T. Mikasa, pp. 115-130. Wiesbaden: Harrassowitz. [available as a PDF from the author at ancientgrains.org]

 

Further description of numerous variations on flotation systems can be found in Pearsall (2000) Paleoethnobotany, 2nd edition. Academic Press, pp. 14-97.