Trench art has a long history, though it didn't get that name until the Great War.
The style goes back as far as when there was a sharp point, and something to inscribe/decorate as a was of passing the time.
One of the first styles was likely done on horn - decorative powder horns are a good example, and horn was ubiquitous.
Metal was a different matter, and often required more tools than a mere scriber.
But I digress...
During the Great War, shell casings would mound up behind the artillery emplacements on both sides of the line, and were was to get, so soldiers would scratch their names, units, and battles, and sometimes scenes, but it was the rehabilitation hospital where they'd get ever more ornate, as a pass-time, and way to channel energies.
They'd develop an assembly-line production - depending upon who could perform what after his injuries.
The shells were filled with sand, and capped with poured lead - sand-filled casings ensured that there'd be uniformity during the shaping process - the design was laid out, the paddles were formed that would crate the flutes, and snips were on hand to do any of the decorative work after the main design has finished.
Then, the work was begun, moving from one soldier to the next.
At the end of the line, backgrounds were stippled, the lead re-melted and the sand poured out, and anything that needed to be cut and bent was done - the whole thing was then polished with sand and usually varnished.
That varnish would later darken over time - accounting for many of the black ones found, but when they left for sale, they were shiny and bright, unless they weren't, because they could also be acid-etched...
Post-WWI, French and Belgian citizens continued the practice to take advantage of returning veterans and the desire for a souvenir.
Many things were made - match box holders, candle sticks, desk sets, shot glasses, lighters, ash trays - pretty much anything a tourist might want could be made, and was offered.
The art form remained as late as Korea, but little of that is seen, though 'Sweetheart' grips are considered so.
These things were produced by all sorts - including POWs, who produced some outstanding carvings on the M1911 grips, and the British REME shops, who had access to better tools and materiel.