The Eprouvette
feature By: Marc Davison | August, 20
We are all aware of the modern technology to quantify gunpowder performance such as chronographs and pressure transducers, but have you ever thought about how black powder performance and quality control were measured before our current technology was available? This article is about a device called the “eprouvette” which, literally translated from French, means “test tube,” but in relation to firearms it is also simply referred to as a “powder tester.” Testing devices in many forms for measuring the strength of black powder were in use from the late 1500s until the mid -1800s.
Black powder has been around for hundreds of years. Formulations, sources and purity of ingredients, as well as manufacturing processes have varied considerably over that period. In the early 1600s, the Europeans realized that due to this variability in manufacturing, some quantitative method of testing powder strength was essential to replace the visual observations and open burning tests, not only for small arms but also for artillery. The Art of Gunnery from 1647, by Nathanael Nye, contains a lengthy description of the characteristic used for both visual inspection as well as open ignition tests for black powder. The need for a better system led to the more refined development and deployment of the eprouvette.
The general principle of the eprouvette is to ignite a measured change of black powder and measure the resulting force through a number of means. The eprouvette is generally regarded as a European invention from a time when Europe was the center of technology, and the state of the art around arms and gunpowder was undergoing a period of rapid transition. However, when the European settlers came to North America, the making of gunpowder had already become somewhat standardized, and although further refinements would occur, the emphasis shifted from obtaining a reliable powder to perfecting firearms; thus, the eprouvette was not commonly encountered in America.
Origin and Forms
The first recorded powder tester, described in a 1587 British book by William Bourne, consisted of a canister with a lid held down by a serrated vertical latch, which provided resistance when the powder charge was ignited. Other types of black-powder testers were developed over the centuries, but two types seemed to persist as the standards.
For military applications, the fixed mortar eprouvette, which originated in the mid-1600s, and persisted through the 1870s, consisted of a small mortar with a fixed trajectory (typically 45 degrees), which fired a known quantity of powder and a known weight projectile. The testing was done in an open field so the distance of travel of the projectile could be measured.
The second most common type of eprouvette was the handheld type, which was used to test powder performance for small arms and was viewed as a more practical device at the time. By the mid-1800s, the eprouvette had all but disappeared as a relevant testing device for black powder, largely because it was recognized that performance in an eprouvette did not translate to performance in a gun/artillery, and because better processes and devices were in use. Nitro powders were also coming onto the scene.
The Handheld Eprouvette
Surirey de Saint Remy in 1697, was probably the first to describe the handheld powder tester, one of which was the pistol type, and he is generally regarded as the inventor of the pistol eprouvette. This type of powder tester was the most commonly used type in France and throughout Europe for the next 100 years. The handheld eprouvette was essentially a proofing instrument for fine-grained powder. Its three primary uses were (1) allow the purchaser of new powder, whether a local source or while traveling, to determine its relative strength compared to a known powder, (2) allow for testing the relative strength of different powders of the same granulation, and (3) to test the uniformity within the same lot of powder.
One of the major issues recognized with the handheld eprouvette, was that the open combustion that takes place in the powder chamber does not produce the same result as powder confined in a barrel. Most of the powder combustion takes place as the lid is being lifted off the powder chamber, thus most of the powder would not burn completely before blowing open the chamber. The evolution and general use of the electric chronograph in 1890s, by le Boulenge, and T.J. Rodman’s (U.S. Army) development of the pressure gauge all but ended the utility of the handheld eprouvette.
In the book by Kempers, which is generally considered to be the authority on eprouvettes, he classifies the handheld eprouvettes using five attributes, the two most notable are 1) ignition type and 2) type of resistance principle for measuring the powder force. The ignition types generally followed, but lagged slightly behind the firearms of the period and consisted of hand-ignited, flintlock and percussion. The hand-ignition type was the earliest and the most common through the early 1500s, while the flintlock-ignition type was the most prevalent and persisted from around 1600 to the mid-1800s. The percussion-type was the least common and was most prevalent during the mid-1800s.
Three resistance mechanism types are described, and the two most common are friction and compression. The friction-type consists of a graduated, serrated wheel, which turns with the force of the ignition and works in combination with a spring that provides resistance. These mechanisms were used from the earliest eprouvettes in the late 1500s, until the end of the eprouvette production period in the late 1800s.
The compression-type generally recognized as a French design, consists of a large graduated V-spring, which is compressed by the force of the ignited charge. This mechanism was introduced in a handheld form around 1780, and also persisted through the remainder of the eprouvette period. The DuPont family imported this type from France and used it in their U.S. powder mills from 1800 to 1820.
All handheld eprouvettes were individually made and had different spring strengths and scale graduations, so it was important to use the same device for all testing. In addition to these two primary attributes, Kempers’ classification system also considers the eprouvette shape (pistol-shaped, vertical, or straight), overall size, and whether the device is single or multi-purpose. Of the many hundred eprouvettes catalogued and studied by Kempers, flintlock ignition systems comprised 52 percent, hand-ignition 34 percent and percussion 14 percent. Of the resistance types, the friction mechanism was observed almost 90 percent of the time. Of the grip types, the pistol-type devices are by far the most prevalent with the straight grips being the least common.
The Mortar Eprouvette
Throughout the first half of the seventeenth-century, powder composition, formulation, ingredient quality, and grain size varied widely by manufacture and country, thus firearm and artillery performance was not consistent. The “lawlessness in composition and grain” as stated by Hime was due to the lack of any method to measure the comparative strength of gunpowder, thus the impetus for the development of the early gunpowder testers, and the first mortar eprouvette around 1647, most of which occurred in France. In 1686, King Louis the XIV complained of the variety of mortar eprouvettes in use and directed that all further powder testing be done in a government standard mortar, using a 60-pound ball and three ounces of powder, and which had to reach a distance of 320 feet to be an acceptable powder. This design persisted throughout much of Europe well into the nineteenth-century and by the early 1700s, most countries had generally settled on powder composition with the primary difference being the granulation. Most European countries were also using mortar eprouvettes through the late nineteenth-century for powder testing, although the mortar configurations, specifications, and proof charges differed.
In the United States, Congress passed a resolution in August of 1776, requiring that “gunpowder be approved by public inspectors as to its quickness in firing, strength, dryness, and other necessary qualities. Mark each keg approved with the letters USA.” Initially “spring-retained” testers were developed and employed, but by the 1830s, the fixed mortar eprouvette came into regular use by the U.S. military. In 1839, Ordinance Regulations were published with specific instructions for testing all gunpowder using the regulation eprouvette mortar. These mortars were to have a fixed trajectory (typically 45-degree), and fire a 25-pound ball using one ounce of black powder. Two charges were fired in the eprouvette from each factory-fresh 100-pound cask, and if the distance variation exceeded 20 yards, a third charge was fired. The average of the charges for a cask had to be at least 225 yards or the cask was rejected, and if the entire lot had an average of less than 250 yards it was rejected. A further test required a 10-grain sample, openly burned on a copper plate, could leave no “spots for foulness.” The proof range and date were stamped on the end of the cask. Subsequent periodic re-proofing was required of all stored powder. Powder with an eprouvette proof range of less than 180 yards was not to be used for “service charges” but rather for “firing salutes and blank charges.” If the range was less than 150 yards the powder was considered unserviceable. Powder used in any firearm proofing had to have a proof range of not less than 250 yards.
Further testing at the U.S. Ordinance Department using 17 identical eprouvette mortars confirmed that this particular testing methodology was inaccurate as identical weight balls and identical weight charges from the same powder lot produced proof distances that would vary by up 35 yards. By the early-to mid-1840s, the methodology had shifted from the mortar eprouvette to the French design ballistic pendulum, which was much more accurate and consistent, and based on the recognition that the mortar always favored the finer granulation powders due to their faster burning rate. By 1861, the pressure gauge was developed and in use. It was at this time that the relationship between grain size and velocity/pressure began to be better understood and quantified. The result was to better match projectile weight with powder granulation.
Conclusion
While the average shooter does not run across an eprouvette every day, they were an important part of black powder history.
Although they are rare today, they do surface occasionally in various auction venues, but prices are high. For the adventurous, Dixie Gun Works still sells an Italian replica, should you run out of other things to do. S
References:
• Lewis, Berkeley R. 1956 - Small Arms and Ammunition in the United States Service
• Hime, Henry W.L. 1904 - Gunpowder and Ammunition
• Kempers, RTW 1998 - Eprouvettes
• Muller, H.G. 1973 - A Brief History of Powder Testers