Rebuilding Coehorn Mortars
Last year when starting to plan for this new group, the intent was to have a light three pound cannon as our first gun. However while we were waiting for that gun to be built we stumbled upon a Coehorn mortar for sale online and it was only two towns away! We know that Henry Knox brought 4 of these Coehorn mortars back to Boston from Fort Ticonderoga in 1776 so we decided to take a ride and check it out.
What is a Coehorn mortar you ask? To answer that we need to look all the way back to the very beginning of the 18th Century and introduce Baron Menno van Coehoorn who was an Engineer with the Dutch military at that time. He participated in many campaigns among them the Nine Year's War, or War of the Great Alliance from 1688-1697. It was in these conflicts that Coehoorn devised new methods and technology related to siege warfare and in 1701 he demonstrated his new weapon, what we know now as the Coehorn mortar. The concept of a mortar in its simplest form is a weapon that fires a shell, often explosive in a high arching trajectory. This high arc allows it to defeat defenses by flying over walls, redouts or other fortifications and exploding in the air or on the ground once it lands. The mortar itself is cast of brass and was fixed to a large wooden block at a set angle of 45 degrees. Adjustments to this angle could be made by using shims and various powder charges, we'll look at the ammunition at a later date.
Flash back to 2023, we took a ride to that local mortar that was for sale and took it home with us, as you can see in the photograph it was looking a little rough. As you can see n the photograph it was looking a little rough. The bed was constructed of three separate pieces of pine, the trunnions were not set all the way into the bed and all of the hardware was incorrect. We initially wanted to rehab the original bed, but we decided that it was too far gone, and incorrect to be used.
Shortly after we obtained the first mortar, a second mortar popped up on an online forum, this one was further away in Maine, but looked identical to our first piece. As it turned out these mortars were cast by the LaPan Foundry in New York, which made many types of iron and brass reproduction cannons years ago. Their pieces were of good quality and most of them are still around today being used by various museums and reenactment groups. We were pleased to learn that the the second mortar was produced at the same time, or immediately after our first one was, and are so marked on their trunnions as No. 34 & 35. We have since referred to them as numbers 4 & 5. Like the first mortar the second gun was in even rougher condition, while both of the brass barrels were fine, this one also needed its bed to be completely replaced.
We then began to plan for the rehabilitation of these pieces. The first step was to polish the barrels up. Using a combination of Brasso, Flitz and 0000 steel wool the tubes once again began to shine!
From there we had to search for wood. Getting this large dimension lumber was not as easy as we thought it may have been. Calls to a couple of lumber yards yielded no results, only after posting on a local woodworking FB group did we get lucky. Traditionally these beds are made of a hardwoord, usually oak or something similar. While we were unable to find oak we did get two blocks of Hard Maple, and were able to get them cut to the correct dimensions (28"x14"x10"). We got them into the shop and weighed them, they both were around 125 pounds! We lifted them off the ground so air could circulate freely, and let them sit for 4 months in a dry basement. Once they were dried they had experienced some cracking, which was to be expected, thankfully nothing that effects the integrity of the bed and they now weighed about 90 pounds, much less but still quite heavy!
Once the new beds were dry we set out to carve out the trench that the trunnions sit in and angle the corners. We have seen several examples of mortar beds at various museums and historical sites and there is some variation in how these beds looked. All had the sharp corners sanded down and some were done more elegantly than others, it is our interpretation of these beds, that they are much less involved than cannon carriages and given the abundance of hardwoods in the colonies in the 18thC that they were probably somewhat disposable items. We chose to just sand down each edge so it wasn't a sharp 90 degree angle and leave it at that.
We used a combination of modern tools, drill and routers as well as an old fashioned hand chisel to carve out the "trench" that the mortar trunnion sits in and the angled "ramp" that the mortar chase leans upon. After some trial and error and repeated adjustment we got both mortars to sit in the beds at the correct 45 degree angle.
Once the beds were complete they received a primer of the correct linseed oil paint. This mixture was 70% linseed oil and 30% linseed oil white paint. This paint/oil mix took a full week to dry!
Once the primer was dry the beds received a coat of linseed oil based grey paint, mixed with some white to achieve a dark grey lead color of paint, we based our paint tone off of the great artillery on display at Fort Ligonier in Pennsylvania. Once the paint was dried we went to our local blacksmith Dean at Rock Village Forge to get all of the iron hardware needed to assemble the beds. Once we picked up the hardware we set to assembling the blocks and then painted the iron hardware!
Once the iron hardware dried we painted No. 4 & No. 5 on the beds so we could identify which gun was which and that was that! The culmination of a 6 month long project had come to an end. We feel that the final product represents these important pieces well and we are very happy with how the project turned out!
Thanks for reading and stay tuned for future projects!
Authors note, the title "High Angle Hell" refers to a phrase used by modern US Military personnel when referring to mortars. The author spent 8 years in the US Army National Guard as a qualified 11C Indirect Fire Infantryman and worked with 60, 81 and 120mm Mortar Systems.