Take thou also unto thee the chief spices, of flowing myrrh five hundred shekels, and of sweet cinnamon half so much, even two hundred and fifty, and of sweet calamus two hundred and fifty, and of cassia five hundred, after the shekel of the sanctuary, and of olive oil a hin. And thou shalt make it a holy anointing oil, a perfume compounded after the art of the perfumer; it shall be a holy anointing oil.Exodus 30:22-25
In laymen’s terms this is giving the recipe for one part myrrh, half part cinnamon, half part calamus and one part cassia. Then a hin of olive oil which works out in weight to be one part. All is fine until one tries to make this and quickly discovers one part of oil is not going to cover 3 parts of herbs. Some seek to find an answer in the last line, that it is compounded after the art of the apothecary or perfumer. Some will assume this is distillation where each herb is boiled and then condensed into an oil and the oil is mixed with one part olive oil. Others will use an alchemy approach and soak the herbs in a strong grain alcohol and then subject the extract to heat until a thick residue is left. Then it will be mixed with one part olive oil. There seems to be no solid way of making this oil.
We need to consider at the time, the tribes of Israel were nomadic and traveled. It’s pretty safe we can rule out the alchemy process as grain alcohol likely wouldn’t have been readily available, nor the tools to distill. Primitive distillation was possible, but we know the oil used drenched Aaron down to his beard. The amount of herb would not produce the quantity of needed extract to cover Aaron from head to beard. So how exactly was this oil made? The answer lies in a work titled “Josephus”.
Moses now purified the tabernacle and the Priests: which purification was performed after the following manner. He commanded them to take five hundred shekels of choice myrrh; an equal quantity of cassia; and half the foregoing weight of cinnamon and calamus; (this last is a sort of sweet spice;) to beat them small, and wet them with an hin of oyl olive. […] to mix them together, and boil them, and prepare them after the art of the apothecary, and make them into a very sweet ointment.Josephus
If we look closely we are to wet them with an hin of oil, then mix them together and boil them. This would indicate each ingredient gets one part oil for a total of 4 parts (one for each ingredient). When this oil is made, it absorbs all the herb perfectly and seemingly to capacity. It then is boiled (over lower heat as not to burn the herb) and strained to produce an historical account of the holy anointing oil.