Brown powder

Brown powder or prismatic powder, sometimes referred as "cocoa powder" due to its color, was an explosive-based propellant used in large artillery and ship's guns from about the 1870s. While similar to black powder, it had slower, gentler, burning rates; and used less-reactive fuel ingredients. Further modifications of its burning rate were achieved by shaping the powder grains into prismatic shapes, typically single-perforated hexagonal or octagonal prisms. They became obsolete as a propellant due to the introduction of nitro-explosive propellants such as Poudre B, in France, and later by cordite, in Britain. These new propellants produced less smoke, particularly less black smoke.

Development
Brown powder, also known as prismatic powder, and sometimes referred as "cocoa powder" due to its color. It was a development of black powder and was designed to produce with a slower (and therefore gentler) burning rate. These differences in burning rate were achieved by incorporating fuel ingredients that were in a less-reactive state that those used in gunpowder. Pulverized and fully processed charcoal (elemental carbon) in black powder provides its distinctive black color, while its replacement with a different substance produces a more reflective powder, hence its name "brown powder". Further modifications of burn rate were achieved by shaping the individual powder grains, often into prismatic shapes such as single-perforated hexagonal or octagonal prisms.

United States
Large-grained powder, made in the traditional way as flat sheets but screened to larger sizes, was introduced in the 1850s by U.S. Army Major Thomas Rodman for his large-calibre cannon. In 1875 Lammot du Pont invented Hexagonal powder for large artillery, which was pressed using shaped plates with a small center core; about 1.5 in diameter, like a wagon wheel nut, the center hole widened as the grain burned. By 1880 naval guns were using Hexagonal 1 in in height. Very large grain powders, being subject to defects in manufacturing, did not completely remove the danger of overpressure, as demonstrated in the 1880 accident on the Italian ironclad Caio Duilio, which involved powder made at the chemical works at Fossano.

Europe
In 1884 the German Rottweil Company developed Prismatic Brown Powder (PBC), which was also adopted by the Royal Navy in 1884. It retarded burning even further by using only 2 percent sulfur and using charcoal made from rye straw that had not been completely charred. It was pressed into prisms with a central hole, similar to the 1.5 in DuPont Hexagonal.

The French Navy instead developed the Slow Burning Cocoa (SBC) powder, which had grains of about 3.1 mm; still only 40% of it burned, the rest was ejected as heavy black smoke. All these were rendered obsolete in 1886 when the first smokeless propellant, the guncotton-based Poudre B was developed, triggering rapid development of other smokeless compounds. Some Royal Navy ships like the Royal Sovereign-class battleships were commissioned with the already obsolete SBC-based guns, but the subsequent Majestic and Canopus-class battleships were the first to adopt Cordite Mk I.

Technical considerations
For pure explosive damage, high burn rates or detonation speeds (and accompanying brisance) are generally preferable, but in guns and especially cannons, slower-burning powder decreases firing stresses. This allows for lighter, longer (and more accurate) barrels with associated decreases in production and maintenance costs.