In France, decorative copper, brass, as well as silver, are seen as metals to be maintained in their metallic state. This tradition can be seen on the French eBay where the description in the "artisanat de tranchees" section often includes the comments: "to be cleaned," "to polish back to a beautiful shine." Of not much consequences for the design since most shells have not been polished for decades.
Polished shells come in hues from buttercup to pinkish depending of the amount of copper in the alloy.
Some may be partly polished to enhance the design against a darker background of patina or black paint.
In England, it seems that shells have been thoroughly polished or varnished. Unfortunately this zeal of perfectionﾊslowly lessened the visibility ofﾊ the very fine engraving.
In the US, the "European Museum Gold" is not appreciated by militaria collectors. An even patina, acquired by oxidation along the years, without spots nor blemishes, is a big plus. One may admire the superb bronze satin sheen of some commemorative shells.
Some might have been varnished to avoid the chore of polishing, or nickel-plated for a grayish silver shine or chrome-plated for a spectacular "car bumper of the fifties" mirror effect.
On the "headstamp" (the base), are engraved the caliber, the manufacture or place of production, often in acronyms, and at least the two last digits of the year of production.
Here are a few examples:
75 DE C. A.TE. 30SL.18 C. =ﾊ 75 mm DE Campagne (field gun)ﾊ Ateliers de ToulousE 1918ﾊ.
PATRONENFABRIK KARLSHRUE ﾊJuli (July) 1917 Sp (Spandau) 255.
REMINGTON UMC (Union Metallic Cartridge)ﾊ 17 (1917)?- 18 Pr (18 Pounds) P E Z (manufacturer) 14 9 17 (Sept. 14, 1917)ﾊ
More examples on the pictures.
Shells used for Trench Artﾊare mainly:
The 75 mm French and later American, the 77 mm German, the smaller 37mm (in diameter), the 13 and 18 Pounder (Pdr)British. A few 105 mm shortened, that otherwise are large and heavy, therefore impractical to handle, to bring home.