Pellet powder was a large grain powder designed to be used in larger guns. In our above example, each pellet is a formed cylinder of black powder about 1.25 inches in diameter with a hole in the center.
Sir John Anderson of Woolwich arsenal in England invented a machine in the 19th century for their manufacture, the details of which are below:
A machine for making pellet powder invented by Sir John Anderson. Click on the image to enlarge. Public domain image.
It consists of a disk of about 6 feet diameter (the pressing table) which revolves about one of the columns. The disk has teeth all around its circumference, which allows it to be rotated by means of a pinion and handle mechanism. The disk has four round metal plates placed symmetrically, about 2 inches thick and 1.5 feet in diameter. In each metal plate are drilled about 200 cylindrical holes of about 5/8 inch diameter. Above each plate is a movable covering-plate which can be pressed tightly against it, and into each of these 200 holes a small plunger enters, which goes through the bottom part of the disk and can be lifted from below by a hydraulic press.
Two opposite plates are always pressed at the same time. As soon as the movable plates are lifted, the molds are filled with meal powder, the plates are cleaned and excess powder wiped off, and the movable plates lowered and fixed so that they close the holes on the top. Then the plungers are pressed into the molds, causing the layer of powder to be compressed to 5/8 inches in height. After this, the movable plates are lifted and the plungers are pushed further into the holes, thereby pushing the formed pellets out of the mold holes.
Click on the image to enlarge. Public domain image.
After the pellets are pushed out, the disk is then rotated for a quarter turn and the pellets are taken off the two mold-plates. Meanwhile the same operation is then carried out with the other two plates.
The pressure applied to the powder by this machine is about 0.5 tons per square inch. The pellet formed is shaped like a cylinder with one or both bases having a hollow in the middle in the shape of a blunt cone. The size of the pellets made by this machine are 5/8 inch diameter, 5/8 inch height and depth of the hole is 1/4 inch and each pellet weighs about 100 grains.
In America, the Du Pont powder company made a hexagonal pressed pellet powder, which looks like two truncated hexagonal pyramids connected by a cylindrical layer of powder.
Du Pont Powder. Public domain image.
This powder was made by the following process: A lower plate in which a number of pyramidal holes were cut was covered with powder and a second similar plate was laid over it and then pressure was applied. Depending on the thickness of the layer of powder, the cylindrical part connecting the two pyramidal halves will be thicker or thinner. After pressing, the cake is broken, this causing the grains to break off on the edges of the cylindrical part.
In Italy, they made compressed pellets in cubical form, sold under the brand name "Fossano Powder", because it was first manufactured in a gunpowder factory at the town of Fossano in northern Italy. Fossano powder is a type of "Progressive Powder" and was invented by Colonel Quaglia (the factory director) and his assistant, Captain de Maria.
Fossano Powder. Public domain image.
The manufacture of Fossano powder was done in multiple stages. In the beginning stage, meal powder was pressed into cakes of density about 1.79. Each cake was then broken up into irregular grains of about 1/8 to 1/4 inch in thickness. Then grains were then mixed again with a certain quantity of meal powder and then pressed into cakes again, with a density of 1.776. This second cake was then broken up into cubes. Therefore, each cube would be composed of powder pieces of higher density enclosed in a powder material of lower density, sort of like raisins inside a plum-pudding. The idea behind this was that due to the differing densities of powder, more gas would be produced after the powder has been partially burnt, than at the start of ignition of the powder, leading to the 'progressiveness' of the explosion (which is why it is called a "progressive powder"). This allows the pressure on the projectile to be maintained during its course in the bore and possibly increased while it is moving away.
Pellet powders burn slower than other ordinary large grained powders due to their larger grain sizes and is therefore less violent in action. Experiments in England showed that these could produce muzzle velocity greater than ordinary large-grained powder with peak pressure hitting about half that of large-grained powder.
Pellets are still available today for black powder enthusiasts:
Pyrodex 50/50 grain pellets/
Click on the image to enlarge.
Click on the image to enlarge
The above images show modern pellets available today in many sporting goods stores. However, these are made of black powder substitute, not original black powder. Black powder substitute is less sensitive to ignition than real black powder and is more energetic.