Some ores are
sent to the smelter with from 5 to 15 per cent. of adherent water. In
these cases it is best to spread out the sample, and taking equal
portions fairly at regular intervals, weigh into a Berlin dish 20 grams.
This should then be dried over a sand-bath, or if the ore is likely to
be injured by excess of heat, over a water-bath until the weight is
constant. The loss of weight multiplied by 5 gives the percentage of
water present.
Example:--
Weight of dish + wolfram 32.67 grms.
" " dish 12.67 "
-----
" " wolfram 20.00 "
" " dish + wolfram 32.67 "
" " " dried 30.15 "
-----
" " water 2.52 "
2.52 ? 5 = 12.6 ~12.6%.~
There are other ores which are not apparently wet, but in the state
called "air-dried." It is easier to take fair samples of these, and,
consequently, it is not necessary to use so large a quantity as 20
grams. But with a smaller quantity, extra precautions must be taken. All
dry solids at ordinary temperatures absorb moisture from the air. The
amount varies with the nature of the material and with the quantity of
surface exposed. Light bulky powders absorb more than heavy ones,
because of the greater condensing surface.
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