For compounding ointments a slab of marble or porcelain and a steel or horn spatula are generally employed, but if the ointment contains an insoluble powder the mortar and pestle must be used. The slab is all right for experienced operators, but even they cannot be sure that they get the powder equally uniform by this means. The better plan is to put the powder in a mortar, which in cold weather should be warmed with boiling water; if the basis be not too hard or too bulky, the heat of the mortar suffices to liquefy it sufficiently for thorough mixture. The powder is rubbed with a little more than its own weight of the basis until a perfectly smooth mixture is obtained, no appearance of grit being evident to the eye, or to the finger and thumb when a little is rubbed between them. The rest of the basis is then added in portions until the whole is thoroughly incorporated.

Extracts, balsams, or any fluid or semi-fluid should be added in such a state as is best fitted to produce a perfectly homogeneous mixture. In regard to powders a small proportion of the basis is, in the majority of cases, sufficient to reduce them to a fine enough state of division ; but in some cases, and often with extracts, a preliminary treatment with some medium, such as oil, water, or spirit, is necessary. Whatever medium is chosen, it should not in any way interfere with or affect the medicinal properties of the ointment. Watery extracts should be rubbed down smooth with a little water before being combined with the fatty basis, and spirituous extracts with a little diluted spirit. Soluble salts, such as perchloride of mercury, sulphate of zinc, and nitrate of silver, which are likely to crystallise, are best rubbed smooth with a little oil or dissolved. Very soluble or deliquescent salts, such as carbonate or iodide of potassium and chloride of zinc, are best rubbed down with a little water, if this will not favour interaction with other ingredients.

Tartarated antimony should be mixed dry with the ointment-basis.

Ointments are prescribed occasionally whose ingredients must be melted. In such cases the hardest ingredient- e.g., wax or hard paraffin- should first be melted by the heat of a water-bath or well-regulated gas-flame, the other fats or oils next added, the mixture stirred until clear, and then strained through cheese-cloth or fine muslin into the pot. If the mixture has not been overheated, the contents of the pot may be allowed to cool without stirring, as it has been shown that ointments so prepared solidify uniformly and without becoming lumpy; but if the ointment contains any ingredient which may separate on cooling, it is advisable to pour the strained mixture into a warm mortar and triturate constantly until the mass becomes pasty. Liquids are frequently added to ointments in order to impart a cooling property to them, as in the case of cold-cream. In the preparation of these ointments a quasi-emulsion has to be made, and circular stirring towards the dispenser is important. The action must always be slow, and larger quantities of a liquid can be incorporated thus than the inexperienced would deem possible. A striking instance is shown in incorporating liquor plumbi subacetatis with lard.

Not less remarkable is the effect on colour: otherwise dark ointments can be rendered nearly white, and with very faint indication of the original colouring ingredient. This is the case with various shades of brown, yellow, or greenish vegetable colours, the exception being with regard to the partial decoloration of certain chemical salts, such as red oxide of mercury.

Order of Mixing is sometimes as important in the case of ointments as in mixtures, e.g.:

This ointment retains its pink colour if the subcarbonate is rubbed down with a little lard, but if dissolved in a few drops of water, the final addition of the liq. antim. terchlor. produces a brown colour, due to the formation of ferric hydrate, iron always occurring in commercial samples of 'butter of antimony.' In a fatty medium the incompatibles are slow to react. This is further instanced by the fact that tannin ointments may with impunity be made with a steel spatula, as no blackening occurs unless an aqueous ingredient is present.