Or HaChaim on Exodus 30:35/אור החיים על שמות ל׳:ל״ה

1ממלח טהור קדוש. Salted, pure, holy. It appears that the meaning of "pure" refers to colours. The incense was to be of a uniform colour throughout. Seeing that it consisted of many different spices and a quantity sufficient for 368 portions of it was prepared at one time, it would most likely have had a mottled appearance unless the Torah had directed that it should be mixed in such a way that its spices would merge into a single uniform colour. Even though it possessed a uniform colour after having been thoroughly mixed, if someone examined it carefully he would notice that the uniformity was not due to all of its components being of the same colour. Such uniformity would not qualify for the definition "pure." The words: "salted, pure" mean therefore that it was to be so finely ground that even upon visual examination from close quarters its colour would appear to be uniform. This required expertise on the part of the people who pounded it into small particles.