Join Date: Sep 2018
Location: Limousin France
Oh dear. This could get very technical and involve wide digressions. So, in a short version.
Corning (making the gun powder into grains) used to be done by drying the gun powder into 'cakes' and then rubbing them through sieves or graters into irregular grains. It was only later with improving mechanical technology that the powder was pressed under high pressure in hydraulic presses into rock like cakes that had to be broken up into hard grains which were then sieved into size grades.
The older soft grains were liable to crumble in transport and storage. In 17th century Sao Tome they were probably getting soft grains so the governor was asking for either the cakes, un-sieved, in waterproof containers or for the ingredients to combine and corn in Sao Tome.
I think it quite possible that Mexican powder was of the soft corned variety in our period.
It was, and is, quite normal for the finer dust powder left in processing to be reintroduced into the next batch and damaged powder also. If too damaged, especially by damp, gun powder would be reused as a source of saltpetre for new production.
To me all of this points to Mexican production being of low quality meal powder for the civilian market which was soft corned for firearms use.