Japanned tinware is not just painted black but it is a glossy surface coating that is heated to cure.  You can japan tin, leather, wood, papier mache and japanned felt was even used for roofing. The coloring can be blue, yellow, red, brown, etc but the most common is black.  The black coloring can come from charred vegetable mater, lamp black, or asphaltum.  Asphaltum is also called Bitumen or Judean Pitch. 

Asphaltum was used by Egyptians in embalming, which they called numia mineralis.

In America asphaltum was discovered in Utah and Samuel Gilson marketed asphaltum as Gilsonite

Instructions for Japanning can be found in John Stalker and George Parsons A Treatise of Japanning and Varnishing 1688  and Robert Dossies The Handmaid to the Arts: In Two Volumes 1764.

The Handmaid to the Arts: In Two Volumes 1764 talks about the use of a primer before japanning.  This priming does make a difference when it comes to flaking of the material.

There is a vendor selling Black Japanning Asphaltum paint that says do not stove under any circumstances which makes me think it nothing more than paint and not a historic product. 

The heating of Japan lacquer causes it to harden.  Even leather is heated when making patent leather as descibed by  Arnold James Cooley, John Cargill Brough in Cooley's cyclopaedia of practical receipts, processes: and collateral Information 1864 and The Artist's Assistant: Or School of Science; Forming a Practical Introduction to Polite Arts 1801 says "every degree that can be applied... tends to give it a more firm and strong texture"

English Mechanics and the World of Science, Volume 34 from 1882 says ten to twelve hours at 212 degrees.

After all the work of Japanning then comes the gilding.

This could be done with elaborate stencils like this one from 1856-1876.