Experience, that 's my source. Let 's say you need 8 hours to form a single piece breastplate, with cutting, dishing, rolling edges, and adding holes (hammered by hand & waterpowered hammer). And these are 20% of the work. So you would need !36! hours for grinding and polishing. This is to much. You have so called handfiles and later waterpowered stonewheels for rough grinding and need another two or three steps for a shining (not mirror-like) surface. Andrej Pfeiffer Perkuhn does demonstrations from time to time how to polish armor with leather and abrasive.
Starting on page 77 of āIron Documents. Interdisciplinary studies on the technology of late medieval European plate armour production between 1350 and 1500ā you can read more about polishing. I believe there also was something in āThe knight and the blast furnaceā, but Iām not sure. Reitzenstein, āDie NĆ¼rnberger Platterā, S.711, writes also about polishing mills.
āin fact they COULD have been fasterā¦ā didnāt say they were. But I believe if you do nothing else than polishing steel, you will get really quick, compared to us modern armorers who also have to do all the other work. Practice makes you quick.
Do you want a source for the fact that breastplates take over in the 15 century? Every āZeughausinventarlisteā (= arsenalinventorylist) there is.
Or do you want a source that breastplates in 1403 were made quicker than coat of plates? For this, ask any random armorer you want. You are ALWAYS faster with dishing a breastplate.
Ok, if you want to compare a cuirass and a brigandine, have a look. This is a simple cuirass from 1480, now in the Metropolitan.
and this is a brigandine of similar date, from the āBurgunderbeuteā in Basel, 1470s. (Sorry for the bad quality, the light there was really low, as not to stress the fabric)