Were medieval European paints always flat? Or could they have a glossy, or merely slightly reflective surface? Specifically, I am inquiring about shields, but any evidence of glossy or shiny paint would do.

Thanks for any help!

While Gesso is flat due to the admixture of chalk (unless I am mistaken) many other period paints could be quite shiny, especially those based on milk or hide glue.

it woual all depend on how fine the particals in the paint are.. the different between a matt paint and a gloss paint are how fine the particals are ground up when mixed to make the paint.

also oil based paints do have a littel more shin to them, but it mainly comes down to the partical size

I have made some different paints with historical pigments and mediums.
Pure earth pigments with (linseed) oil turned out glossy
Casein glue paint with chalk (and soot, or green earth) flat
Hide-glue gesso with chalk somewhere in between..

Also remember that different types of lacquer and varnishes where known, so a flat coat of paint could be brought to a gloss using resin-based varnish..

actually I got a copy of Theophilus "de Diversis Adrtibus" just a few weeks ago, it contains 12th century recipies for paints and varnishes (among many other useful things) And one example is:

"Of the Varnish Gluten
Pour linseed oil into a small new pot, and add, vary finely powered, the gum which is called formis, which has the appearance of the most lucid This, but when broken, it yields a brighter lustre. When you have placed which over the fire, cook carefully, so that it may not boil up, until a third part is consumed, and guard against the flame, because it is very dangerous and is extinguished with difficulty if it be raised, Every painting, covered over with gluten, is made both beautiful and for ever durable."

Where "formis" is most likely resin from some coniferous tree species.

i also have that book.. just be careful as a most of his recipes involve doing thing and cooking this which are dangerous to you health... the cooking up of lead for example