Paraffin Ointments

Ointments made with hard and soft paraffins are apt to be granular unless they are very carefully made. Melt the paraffins, pour into a mortar previously well warmed with boiling water, and stir constantly until cold or not at all. The difficulty of producing a smooth paraffin ointment has led to an extension of the manufacture of milled ointments. An ointment-mill is beyond an ordinary pharmacy, but a uniform and plastic ointment is obtained by forcing the cold ointment through a perforated plate or wire gauze equal to a No. 40 sieve. Constituents of an ointment soluble in each other do not separate when allowed to cool without stirring (see page 210). If the ointment contain any powder, rub this up with a little of the soft paraffin before adding the melted mixture. Resin (colophony) is insoluble in soft paraffin, and mixtures containing it must be stirred constantly until they set. Although it lays down no specific rule for the guidance of dispensers, the British Pharmacopoeia in its formulae directs white soft paraffin to be used for colourless ointments, and yellow for others.

Sulphuric acid is employed in bleaching some makes of white soft paraffin, and appears to leave something irritating behind.

Resorcin

Few articles give dispensers so many surprises as resorcin, the dihydric phenol (C6H4.20H) which is so largely used in producing artificial dyes. It is a powerful reducing body, and greedily absorbs oxygen from its environment, becoming changed from its colourless state to rose or dark brown; the following illustrate what may happen in ointments:

The resorcin was dissolved in spirit and added to the other ingredients previously mixed; the product became blue owing to reduction of ammoniated mercury. The resorcin should not be dissolved in spirit. Rub it to powder and mix with half the vaseline, do the same with the ammoniated mercury, and mix the two.

The powders were triturated together and mixed with vaseline. The ointment was of a pea-green colour in ten days. It does not become so if the resorcin is triturated with a drachm of vaseline before adding to the rest.

Sometimes the No. I. combination has liq. carbonis deterg. along with it, in which case a blue colour is inevitable.