A simple machine for bulk processing sediments
1. Introduction
Anyone who has had the task of extracting fragile fish bones from clay or silts will be familiar with the problems involved. The commonly used procedure is to dry the sediment thoroughly and then to soak it in hot water. The disaggregated sludge is then sieved through an appropriate mesh. Most clays can be cracked in this way. The disadvantages are that:
Drying can damage fragile plant remains.
The mechanical action of sieving can also damage fragile specimens.
If a small sieve is used the process is slow, but if a large sieve is used the process is tiring. On the other hand, the use of employed labour makes the process expensive.
An alternative method is to deep-freeze the matrix and then immerse it in hot water. This does not compare favourably with drying but will crack some silts and silty clays.
2. Adjuvants
There are a number of reagents that assist the disaggregation of sediments; these add to the expense of the technique, but can crack clays and shales that water alone will not affect.
1) Quaternary 0
This is a wetting agent used to disaggregate small samples of compacted clays. I have no experience of this, and use automatic-washing-machine powder as a joint detergent and water softener to help with sticky sediments. The use of Quaternary 0 is described by Zingula (1968).
2) Hydrogen peroxide
Used as a 20-60 volume solution, hydrogen peroxide rapidly breaks up clays and indurated silts, particularly those containing manganese salts. It also rapidly oxidises pyrite which in turn can corrode calcitic and phosphatic remains. To avoid this, it has been found that the addition of caustic soda (NaOH) to make a 5% solution is beneficial. This, however, destroys some plant remains.
3) Sodium hexametaphosphate
This compound, in solution, breaks up silts and clays, presumably by breaking up their colloidal state. I find it difficult to dissolve, expensive and relatively ineffective when compared with simple drying.
4) Kerosene (Paraffin)
Thoroughly dried clays and some indurated shales and sands crack very rapidly when soaked for 24 hours in kerosene and then washed in very hot, preferably boiling water. Most of the kerosene can be poured off the matrix before adding the water. The remainder can be salvaged later from above the water, thereby cutting the cost. It is probably the cheapest and most effective of all the methods listed here. Residue obtained by this method should not be treated with hydrogen peroxide.
5) Acid
Calcareous clays can be rapidly disaggregated by hot, dilute (2.5%) acetic or formic acid. Generally, little damage is done to the calcareous macrofauna. A variant of this method is to soak dried clays in a solution (10%) of washing soda (sodium carbonate) prior to treatment in acid. This is a harsh but effective method with clays and weak shales that break into pellets rather than totally disintegrate.
3. Bulk processing
None of these methods, however, solve the basic problem of getting a large amount of sludge through a sieve and extracting reasonably intact remains. This problem was partially solved by Henkel (1966) and Freudenthal (1976) who both used large static sieves and a power pump to bulk process large volumes of silt and clay in the field. This method can be very effective but is quite labour intensive. It is no faster with finer fractions or coarse silts and sands, but is less exhausting than hand sieving. The disadvantages of this technique include the bulk (weight) of the equipment and problems of transporting the pump, sieves, suction and delivery hoses et al. to the site. The main advantage is that the residue is prepared in the field and available for immediate sorting. It is probably most effective where a relatively coarse and bulky residue is being washed from a fine-grained sediment.
4. A new technique
Where it is possible to bulk sample the matrix and the results are not urgently needed, the following machine technique has been found most effective:
The machine (Figure 1) is in two parts; a polyethylene tank and a hinged lid assembly. The lid contains three lawn sprinklers, two oscillating and one rotary, both of which can be bypassed by a hose. The sides and part of the top of the lid are made of wood with a sloping, transparent perspex top. Three taps control the relative amounts of water to each sprinkler. The tank is rectangular and holds 350 litres when full. From the drain at the base a pipe is looped up to form an automatic syphon. By trial and error it was found that 25mm. (internal diameter) was suitable for the drainage pipe (30mm. external diameter). A larger pipe fails to syphon, merely overflowing with the tank in full position. A smaller bore pipe often fails to break the syphon when empty. As the bore of the drain pipe needed is determined by the local water pressure, this must be established experimentally. The drainage rate of the tank depends on both the drain pipe dia