It was anticipated that MARC 3 would encounter a number of deposits containing material which would need to be extracted for identification. In the absence of external facilities and in view of the high cost of commercial machines, a flotation system was built, in conjunction with the City of Winchester Rescue Archaeologist. This machine combines features from two systems, including the ability to treat two samples simultaneously and to recycle the water - yet it cost under £30 to construct, largely through the use of discarded and donated materials
The flotation tank is a 50 gallon oil drum with the top removed, painted with anticorrosive paint and filled with water. A 3 mm. builder's sieve, which allows soil particles to pass through but retains stones and small finds, is suspended 30 cm. below the rim on wire struts. Below this is the bubbler (burner from old gas fire) which is connected to a compressed airline by old garden hose. A spout made from aluminium sheet (from an old table) is fixed with builders mastic 10 cm. beneath the rim (See diagram). On the opposite side, a 12 cm. diameter hole has been cut in the bottom and, in use, this is covered by a 14 cm. diameter disc (from the drum lid) which may he pulled out from the surface with a rope, thus releasing the sludge. The tank is lined with concrete in a funnel shape leading to the plug and the whole affair mounted on beams and concrete blocks to allow a wheelbarrow underneath.
Water pumped into the tank gives an overflow which carried the flot onto three nylon-meshed sieves (1 mm, 0.6 mm and 0.4 mm meshes) made from wood battens and clipped together vertically with cabin hooks. From the sieve the overflow passes through ducting (from an old ventilation system) to a sedimentation trough (from Portway) where it meets the water from a second tank placed in tandem. The combined flow passes along the sedimentation trough, through a 50 gallon oil drum to a 150 gallon reservoir. Being clean, it is then pumped back up to the flotation tanks in discarded garden hose by two salvaged washing-machine pumps.
An obsolete electric tyre pump (given by the G.P.O.) provides air which piped to the machine in steel tubing discarded in recent North Sea Gas conversions. The pump automatically maintains pressure, and the flow is regulated by a string operated valve at the flotation tank.
Paraffin and detergent are added in equal concentrations to the water to provide the necessary froth bed and the dried samples are added slowly, with frequent agitation to break up soil lumps caught on the 3 mm. sieve. After every sample the flot sieves are rinsed and the contents turned out onto paper towelling which is folded and air-dried on racks.
Mounted on the reservoir is a small tank with overflow into the sedimentation system which is used for water sieving and small scale flotation. Very good separation has been obtained by shaking the 3 mm. sieve above and below the surface of a paraffin/detergent/water mixture, generating a froth which holds the charcoal at the surface where it may be skimmed off with a tea strainer. (For small samples, or where money or expertise are limited, this could be a useful alternative to full-scale flotation.)
The machine has been in use for four months and given good results but several improvements could be made.
Jarman, H. N., Legge, A.J., and Charles J. A. 1972.
Retrievals of Plant Remains from Archaeological Site by Froth Flotation.
In: Higgs, E.S. (ed.), Problems in Economic Prehistory. (C.U.P.) p. 39-48.
Williams, D. 1973.
Flotation at Siraf.
Antiquity XLVII. No. 188 (Dec. 1973) p. 288-292.
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