Preliminary Results on the
Experimental Attrition of Flint
The aim of this work is to monitor shingle abrasion rates under controlled laboratory conditions.
The Tumbler:
The attrition experiments are performed using tumblers shown in Figure 1. The University of Sussex owns three double-barrel and one single-barrel tumblers. The hexagonal barrels are fully rubber lined to reduce impact with the metal walls and have a smallest diameter of 20cm, a largest diameter of 22.5cm and are 20.5cm deep, thus having a volume of 7000cm³. The barrel revolts at a rate of ~28rpm.
Abrasion of flint:
Abrasion in the tumbler depends on a variety of factors of which the rock properties are the main interest within the BERM project. Other factors influencing the abrasion of a give rock are the amount to which the tumblers are filled with pebbles, the proportion of water added to the pebbles and the time interval over which the tumbler is run without changing the water and flushing the abrasion products.
Freshly broken flint, as it is provided from the Chalk cliffs, has its edges broken within minutes in the tumbler. With increasing roundness the abrasion decreases but nevertheless is astonishingly rapid (Figure 2)
Figure 2: Change in angularity of a freshly broken flint (5cmx6cm) over time. Note particularly the change of the tip in the lower part of the photos. This flint has been tumbled with eight spherical flints and has lost 8% of its weight during the time indicated.
While it appears to be natural, that angular rock will abrade quickly, well rounded beach pebbles are thought to be very resistant to abrasion.
Figure 3 illustrates the wear of a well rounded rod shaped flint that has been tumbled with seven other spherical flints. The flint shown in figure 3 has lost 6.7% of its weight during the 186:15 hours of tumbling. Altogether this flint had been tumbled for 353 hours resulting in a weight loss of over 13%.
Figure 3: Change in roundness of a well rounded flint
The long term tumbling experiments have shown that the abrasion rate decreases with time (Figure 4). This indicates that the conditions on the beach and in the tumbler are very different, i.e. that the energy level on the beach is much higher, producing a shattered pebble surface. This surface wears more easily than the material beneath thus reducing the attrition rate over time in the tumbler.
Figure 4: Graph showing variations of the attrition rate depending on individual rock properties, the duration of tumbling and the tumbling interval. The pebble in figure 2 is the freshly broken flint in this figure.
U. Dornbusch 26-01-2001