I recently purchased a very large bag of anthracite (2mm-4mm pieces) to make hopper loads with. This coal I bought is sold as a filter medium. To me I think it is exactly what is used by the O scale train manufacturers for the coal in the locomotive tenders.
I have made coal loads before with activated carbon fish tank filter medium, but it does not have the shine and glimmer of real anthracite.
So far for this project I have made a base from carved pink foam, painted black with acrylic, coal on top and treated in the car like I would do track balast (spray wet water (water and dawn), and spray diluted matte medium) . It is drying now. Two things I am watching for are 1. Will the glue dull the shine too much, and 2. How strong will it be.
Glued track balast is not held very strongly and I expect the same on the hoppers, but want to be sure it doesn't come loose too easily with use on running trains. My other home made jobs have held up well, so I am not expecting any problem with the adhesive strength, more so the shine, which I will report back on later.
Looking at the tender coal loads on my 3rd Rail, Lionel, and MTH tenders, I find that the coal is glued in very strongly While it can be picked out, yes I did it, the glue seems stronger than diluted white glue or diluted matte medium, and the coal shines well.
I think the matte medium will be fine, but before I mass produce these cars for my train, I figured I would take a shot here and see if anyone knows and is willing to share what kind of glue 3rd Rail, Lionel, and/or MTH use(s) to glue the real coal in their tenders.