The essence of the problem with the gold recovery proces using mercury was succinctly described in an article in the Sydney Morning Herald in late 1865 as follows ...
“The process of amalgamation has been carried on with great success in various parts of the world with ores yielding as little as half an ounce of gold per ton, wherever those plagues of the metallurgist, sulphur and arsenic, are not present.
Whenever either of these elements exist in an ore – and this is the case with by far the larger proportion of gold ores – they have the effect of tarnishing or “sickening” the mercury, as the miners call it, the consequence being that a large percentage of the gold in the ore is unacted on and lost.
The loss of gold from this cause is very great, varying from 30 to 87 per cent, of the metal present. In some experiments by Readwin 2 cwt. of rich quartz gave hardly any gold by the ordinary process, no less than ten ounces of gold remaining untouched in the tailings.
On this account it has been found impossible to work several even of the richest mines. In practice, too, there are several well known cases where grains of gold were visible in the quartz, and little or none was extracted by amalgamation.”