SCIENCE AND INDUSTRY. 
bark are reduced to the size which presents the greatest surface to the solvent, 
but which does not prevent percolation. Commercial ground wattle bark consists 
of coarse and fine particles, the latter being in a powdered state. If too much 
powdered bark be present it adversely affects the ordinary extraction processes 
by preventing percolation or uniform penetration of the water into the various 
layers of the bark. For ordinary extraction processes an amount of water is 
used which will just cover and yet completely penetrate the bark. 
Powdered bark might well be expected to give the quickest and best results 
when used in a process which secures as complete as possible penetration of the 
water. This desirable result could be secured by any mechanical device capable 
of stirring or agitating the powdered bark, but in that case the amount of 
water used would have to be largely in excess of that volume required to just 
cover the bark. The bark used throughout in this work was that of Acacia 
pycnantha, commonly known in New South Wales as ground Adelaide wattle 
bark. Experiments were carried out. to find, with this material, the difference 
between extraction with finely ground and coarse bark. Results from these 
experiments appear to indicate that under local conditions powdered bark has 
but little advantage over ordinary ground wattle bark, but this does not mean 
that less care can be taken when milling the latter. An ideal condition for 
extraction purposes would be obtained when the fibres of the bark are isolated 
but not reduced to a powder. Proctor, in his book, The Principles of Leather 
Manufacture, supplies much useful information on the grinding of bark. It 
would be difficult to teach our local tanners much about milling bark, but it 
may be said that a mill that is inclined to crush and shred the bark is better 
than one that only cuts and breaks it. Ordinary coarse-ground Adelaide bark 
as supplied to Sydney tanners was used for all the experiments described in this 
paper, and one could not find fault with the work done by the mill. 
When wattle bark is covered with water the latter gradually penetrates the 
particles until these are thoroughly wetted. If the water is now allowed to 
drain away from the bark by gravitation it is found that a considerable amount 
is retained by it and cannot be completely removed eyen under a high pressure. 
Our local tanners follow no definite rule as regards the time allowed for 
any one liquor to remain on the bark, but there is no doubt that in a number 
of cases they exceed to a considerable extent the actual time required for the 
result described above. Reduce this time, which may be called the “ one liquor 
period,” to a minimum, and the number of pits, &c., are kept at a minimum so 
far as this factor is concerned. The time for this one liquor period will decrease 
as the temperature increases and as the size of the uncrushed particles of 
bark decreases. It would also be expected to vary with different barks. 
A number of tanners depend almost entirely on extraction at ordinary tem- 
perature, which is a variable figure, and attains its maximum in summer and 
minimum in winter. 
In order to throw some light on the time for the one liquor period experiments 
were made which indicate that at temperatures of 11° to 15° ©. twenty-four 
hours is-a sufficient maximum time. The higher the temperature the less the 
time required, but good results are obtained at 40° C. in 34 hours, 73 per cent. of 
the total tannins being extracted. If the time is extended to twelve hours at 
40° C. this extraction is increased to 90 per cent. 
EXPERIMENT WITH THE Press Leact Barrery. ’ 
After consideration of the results of the initial experiments, it was decided 
that the “press leach system” was the most suitable one for systematic experi- 
ment. Six jacketed copper vats, arranged in series, composed the battery. The 
capacity of these vats was 5,250 ¢c.c. The water required to cover the dry bark 
was from 4,000 to 4,500 c.c., and each day 2,600 c.c. were drawn. off. 
The vat which received the fresh bark was called the “head” of the battery, 
and the vat containing the spent bark was called the “tail-end” of the battery. 
The spent bark was weighed immediately after it was taken from the battery, 
and it was then exposed for drying purposes. The six vats, for convenience, 
may be referred to as A, B, C, D, E, and F respectively. Spent bark was removed 
and replaced with fresh bark each day (Sunday excepted). Each lot of bark 
thus remained in the battery for a period of seven days. It is found in these 
experiments that determinations of the tannin in the liquors and in the spent 
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