If the area of the cathode exceeds that of the anode the solution is said to grow weaker, and vice versa. This may be remedied in the former case by an obvious readjustment; the latter introduces no difficulty so far as I know except when plating iron or steel.
The student need not be troubled at the poor appearance of the deposit before it is scratch-brushed. Heavy gold deposits are almost always dull, not to say dirty, in appearance till the burnisher or scratch brush is applied. On the other hand, the deposit ought not to get anything like black in colour.
The following indications of defects may be noted--they are taken from Gore. I have never been really troubled with them.
The deposit is blackish. This is caused by too strong a current in too weak a bath. This may be remedied to some extent by stirring or keeping the cathode in motion. The obvious remedy is to add a little cyanide of gold.
The gold anode gets incrusted. This is a sign that the bath is deficient in potassium cyanide. The gold anode gets black and gives off gas. The solution is deficient in cyanide, and too large a current is being passed.
If a bright surface is desired direct from the bath, some caustic potash (say 2 per cent) may, according to Gore, be added, or the articles may be plated only slightly by using a weak current and taking them out directly they show signs of getting dull. By a weak current I mean one of about five amperes per square foot.
The deposit is said to be denser if the solution be heated as directed; but the bath will gild, though not quite so freely when cold.
To gild iron or steel directly, dilute the bath as above recommended some five or six times, add about 1 per cent of potassium cyanide, and gild with a very weak current (say two or three amperes per square foot) in the cold. Frequent scratch-brushing will be found requisite to secure proper adherence.
It is generally recommended to gild brass or German silver in solutions which are rather weak, but in the small practice which occurs in the laboratory a solution prepared as suggested does perfectly for everything except iron or steel. The scratch-brushing should be done over a large photographic developing dish to avoid loss of gold. It is a good plan to rinse the articles after leaving the bath in a limited quantity of distilled water, which is afterwards placed in a "residue" bottle, and then to scratch-brush them by hand over the dish to catch fine gold. When any loose dust is removed the articles may be scratched in the lathe without appreciable further loss.
Silver-gilt articles tend to get discoloured by use, but this discoloration can be removed by soap and water. After long use a gold cyanide bath tends to alter greatly in composition, In general, the bath tends to grow weaker, from the fact that there is a strong temptation to gild as many articles at once as possible.