Gristmills are large buildings constructed for the purpose of grinding grain that's been separated from it's chaff, a process done at garners or granaries. For game purposes, a mill by this name reflects a minimal, simple construction that can be found associated with 1 hammer symbol in a given hex. Larger mills, including windmills, are able to produce greater amounts of flour as well as providing other mechanical and manufacturing uses, can be found in hexes with a greater number of hammers.
The grinding capacity of a gristmill is between 2-5 tons per day, depending on the efficiency of the mill; each mill is checked for its efficiency and can be relied upon to produce that much flour every day. Once the flour is produced, it's sacked and made ready for transport to habitated centres, or it's delivered to local bakeries if those are present. The gristmill is occupied by a well-fed miller and family, supporting a population of 1-9 persons (d4+d6-1).
The mill is built around simple machinery, a rotating vertical wheel that's driven by water, turning a large gear-wheel connected to a driveshaft. This shaft then connects to a smaller gear-wheel that turns a top mill-stone in a circle over a bottom mill-stone. Grains are introduced into the space between the stones and is ground into flour.
Minimally sized gristmills possess square or rounded spaces, 625 sq. ft. in area, with a height of 22 ft. The wheel must be at least 13 ft. in diameter. They require authority-status knowledge to construct. Much larger gristmills with larger wheels are possible, but don't occur in hexes with 1 hammer. Generally, a 2 hammer gristmill would be 50% larger, with an efficiency of 6-10 tons of flour per day.
The price for having grain ground into flour varies depending on the locality, but if a base rate is desired, 1 c.p. per 2 lbs. is recommended. The cost of a 40 lb. sack of flour at a typical European market costs between 16-20 s.p.