How effective would a stone cannon be?

Question

How effective would a cannon made of stone be? Would it actually fire? Would it still be usable after firing? Would it be as effective as a metal cannon? How thick would the walls of the cannon have to be?

Answer 1

Not very. Yes. Maybe. No. It depends.

• Would it fire? Yes. The powder charge would launch the cannonball towards the enemy. There might be problems with accuracy. And the cannon might burst, see the next point.

• Would it fire twice? Maybe. Depends on the strength of the walls, as you noted, and also on the presence or absence of flaws in the stone.

• Would it be effective? No. It would probably be harder to produce than a metal cannon, and heavier.

• How thick would the walls have to be? Impossible to answer. One might err on the side of caution, and make them very thick.

Has it been done?

There is something called a fougasse, which could be considered a stone cannon. Note that they were constructed in place, not mounted on a carriage for battlefield mobility.

Answer 2

In reading for this answer I was interested to see that a comparison of cannon metal strengths was published in the New York Times in 1861. This sort of thing was apparently considered to be of general interest. Which is very cool, and maybe a little sad - not for them, but for us.

There are several different qualities which comprise the strength of a material. Different qualities are important for different uses. An important characteristic for a cannon barrel is tensile strength. Tensile strength is the ability of a material to resist being pulled apart. Tensile strength for a material is often presented with compressive strength which is the ability of a material to resist being crushed together. Stone has excellent compressive strength which is why you can make buildings out of big stacks of it. Stone has weak tensile strength - inferior even to wood - which is why we never see stone in applications relying on tensile strength. In the tables below you will see that metals (except lead!) have tensile strength an order of magnitude greater than stones.

(Below screenshot from http://www.arcraftplasma.com/welding/weldingdata/metalchar.htm)

(Below screenshot from Stone: Building Stone, Rock Fill and Armourstone in Construction)

The units for strength are actually the same: $$1\ meganewton/m^2\ (MN/m^2)\ =\ 1\ megapascal\ (MPa)$$

Of course, there is more to it than that: the article from the NY Times mentions that the softness of bronze makes it hard to maintain rifling in the cannon bore. I recently read The New Science of Strong Materials: Or Why You Don't Fall Through the Floor which was really excellent and well written, covering materials science of exactly this sort. I recommend it for bedtime reading.

I have wondered if there might be a workaround for the stone cannon problem. The thing about a cannon is that if it fails on firing it might blow up and hurt its crew. A fix for this would be to bury it - the earth around it would contain a failure and provide a countering compressive force from the outside. Such a cannon could not be moved laterally and so could only be aimed by altering the explosive charge or weight of the ball - which is worth something.

If you were going to use packed earth to oppose the expansion of the barrel, maybe you could use wooden cannons, manufactured like a barrel. There would be some built in give. The staves would be a lot lighter and more durable than a stone tube. A technology inadequate to cast cannons could probably make barrels. There would, of course, be no rifling or anything like that. But a cannon of this sort would be adequate to heave a bomb over to the enemy.

Wikipedia has an entry on wooden cannons. I am loving this rope wrapped one. I have never read anything about burying the cannon or using packed earth with a nonmetal cannon.

Answer 3

Would it actually fire?

Yes, at least once. You might want to retreat to a safe distance for the test firing.

Would it still be usable after firing?

Depends if you calculated the right powder charge and if the flaws in the stone aren't too great.

Would it be as effective as a metal cannon?

No. Compared to traditional metals for making cannons like brass, bronze, iron, and steel, stone has poor tensile strength (resistance to stretching), poor yield strength (how far it can stretch before it will not return to its original shape), and it's brittle (it will crack and shatter rather than stretch).

In addition, it's highly unlikely that a stone cannon can be rifled or retain it. It will be smoothbore and that means an unstabilized shell will be inaccurate. Its low velocity will further reduce the accuracy; the higher arcing trajectory means range finding will be very important.

How thick would the walls of the cannon have to be?

Hard to tell. Unlike metal melted, cooled, and hammered into (ideally) a consistent state; stone has natural fissures, cracks, and flaws which will be weak points. Many will be invisible to the naked eye making it difficult to judge how thick a given cannon from a given piece of stone needs to be. These tiny cracks and fissures will open a little bit more every time the cannon is fired until it finally bursts.

And it depends on what you want to do with it. There are basically two uses for a cannon: as a kinetic weapon or as a delivery system for something else.

Kinetic Weapon?

A kinetic weapon just means it does its damage by smacking into the target: a bullet or solid cannonball are kinetic weapons. It transfers its kinetic energy to the target. While the shape of the projectile has an effect, a longer, pointer projectile will fly and penetrate better, ultimately it's the kinetic energy that matters.

The problem for a stone cannon hoping to knock down a wall with solid shot is this:

$$energy_{kinetic} = \frac12 mass \times velocity^2$$

What that says is doubling the weight of the projectile will double kinetic energy, but doubling the velocity will quadruple to kinetic energy! Similarly, halving the velocity will drop the energy to a quarter of what it was.

Firing a heavier shell is not as effective as firing a faster shell. Since your stone cannon cannot withstand the high pressures necessary for high velocity, its projectile will be very slow. It cannot fully make up for this with heavier shot: eventually, the shell becomes too big to handle. This is why, for example, tank cannons with the same caliber have wildly different effectiveness. The short 75mm M3 gun used on the M4 Sherman tank had a much lower velocity, and less ability to penetrate armor, than the guns of the same caliber like the long 76mm M1 gun on some later Shermans or the 75mm KwK 42 on the Panther.

Your stone cannon can never make up for its very low pressure and low velocity compared to a metal cannon. It wouldn't make a good kinetic weapon. Firing stone balls with a stone cannon against a stone wall will make a big bang, but probably won't do much damage.

A Delivery System For Something Else

The other way cannons are used is as a delivery system for something else. This is typically some sort of explosive, but it can also be chemical, biological, or even psychological (like leaflets). This is ideal for low-pressure guns because all you have to do is get the payload to the target. You could walk the projectile over to the target and set it off and it would work just as well.

The low velocity means if you want it to get very far you have to fire at a high angle. So you'll have some sort of stone howitzer or mortar firing an explosive shell.

Source

Since it will be large and heavy, this is ideally a siege weapon. Something you set up out of range of the enemy's artillery and isn't going to move around much. To give you an idea of a scale, here's a 13" mortar from the American Civil War.

Source

That's probably made of iron or low quality steel. It weighed 17,000 lbs and fired a 200 lb shell. A stone cannon would be much, much larger.

This could be mitigated somewhat by reinforcing the stone cannon. Metal bands are normally wrapped around the chamber for reinforcement. A siege mortar, once in position, could be packed with additional stone and earth to further reinforce it... though with the very limited flexibility of stone it might do nothing more than providing a pit to contain the shrapnel when it bursts.

Wooden Mortars

If you're looking for a non-metal to make your cannon out of, what is known to work is wood.

Mythbusters famously made a wooden cannon work, wrapped with iron. More convincing is the use of wooden mortars by the Union Army of the Tennessee. Lacking proper siege weapons, they improvised. Reportedly these worked and could fire six to twelve pound shells, equivalent to 57mm and 76mm today. Very, very small for artillery that normally starts at 76mm up.

Again, it will be much lower velocity than a metal cannon, and it will be smoothbore. But it's much easier to bore out, and the flexibility of wood means reinforcing it with metal bands and earth will be more effective.

For more on wooden mortars and improvised siege weapons, try Engineering Victory: The Union Siege of Vicksburg by Justin S. Solonick.

Answer 4

There are multiple, historical examples of artillery/mortars firing stone balls. Wikipedia has an entry (with photos, including some stone balls) showing only the biggest ones. So there's enough of a historical record that these weren't total death traps for the crews working them. I suspect there were many more of smaller caliber: https://en.wikipedia.org/wiki/List_of_the_largest_cannon_by_caliber#Stone_balls

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If you are willing to accept the speculative, a stone cannon is possible -- without being carved from an existing block of stone. Instead, it could be made from two kinds of molten stone. Probably re-melted stone, rather than lava, since we need two different types, one to produce strong, elastic fibers and a second (with a lower melting point) to form the matrix that the tensile fibers are embedded into, as follows.

Basalt fibers are a real, usefully strong, commercially available thing. See en.wikipedia.org/wiki/Basalt_fiber
and
basalt-fiber.com
So, one could easily imagine using basalt fibers, in some suitable matrix, to make a cannon barrel out of stone. We first produce basalt fibers, which have yield stresses and strains very similar to glass fibers.
Some of the mechanical and thermal properties are summarized here: http://smarter-building-systems.com/smarter-building-basalt-faqs/

I posit that an all-stone-derived fiber-reinforced ceramic material would give us much of the strength of the fibers, so long as we:

• make the fibers dominate the volume. Namely get as close to (pi/4 as optimal long cylinder packing, aligned case of course) as we can.

• embed the fibers in another melted mineral, so long as it has a lower melting temperature than the basalt and is chemically compatible at least short term. From http://www.minsocam.org/msa/collectors_corner/arc/tempmagmas.htm
  it's evident that we could use a rhyolitic mineral for the matrix material, for the lower melting temperature. There are alternatives, and Cryolite (Na3AlF6) has a fairly low melting point: http://www.gly.uga.edu/railsback/Fundamentals/HardnessMeltingPlot06.pdf

• The fiber is wound on a removable (or sacrificial) mandrel. The orientation of the fibers can be changed (even on a layer by layer basis) to optimize the strength and shock/fracture toughness.

• The wound fiber is placed in a mold (ideally a sealed, evacuated mold), preferably preheated, and then the molten-rock matrix is poured in.

• After the casting cools, the bore can be ground out to the desired diameter. With sufficiently localized heating (and a sufficiently large caliber to permit access), such a gun barrel could be repaired and maintained.

By judicious casting, a rifled barrel is not entirely beyond possibility.
This would only make sense if virtually no usable metal were available -- but such a world might exist.

Answer 5

Stone cannon balls were used many times. But not cannon for many reasons, not least of which would be cost. They're just impractical for a robust weapon. Of you made one thick enough to handle blasts and greased it every time it might work for a while, but would be hard to transport. If you used a rock in situ it would crack itself to pieces with repeated recoils.

There is a picture here of a ceramic cannon but it probably hasn't been used in warfare although may have been around in a few arguments and strong words depending what it was loaded with.

Answer 6

There is a potential use for a stone cannon.

Stone, as mentioned previously, has very poor tensile strength, chips, and flakes, and has flaws which cannot be easily spotted. Unlike metal, melting it down to remove imperfections is not practical considering the extreme melting temperature of stone compared to metal. Stone cannot be worked as quickly or efficiently either. So, a cannon that is like a traditional metal (or even wood or bamboo) cannon is not worthwhile.

So forget about trying to make a conventional cannon.

A cannon contains the expanding gasses from a rapid burn of propellant such that it forces a projectile out the end of the barrel like a cork out of a champagne bottle. Stone isn't good at safely holding in these gasses in a reliable manner, so what if we invert the paradigm? Make the "cannon" and the projectile effectively the same thing.

Step 1; Create a carefully shaped hole in the ground. Pack the earth. It should be more or less crater shaped, but with a flat bottom and the entire "crater" should be angled as if from an impact at about 45 degrees.

Step 2: quickly grab a big boulder from nearby, shape the boulder roughly so that it has a very shallow "bowl" on one end. Knock off any really extreme points or ends so it is more or less round. You will also need a small groove or channel cut from one outside surface to the "bowl".

Step 3: Set a large charge of gunpowder or whatever you are using as propellant right in the middle of the flat bottom of your angled packed-earth crater. Lay flammable wick or fuse of some kind from the central mass straight out and up one side of the crater.

Step 4: Carefully lower boulder down over the charge so the charge is in the "bowl" you cut in the boulder and the wick falls inside the groove you cut earlier.

Step 5: Light the fuse, run a pretty good distance away.

Step 6: Watch a HUGE boulder go flying onto your enemy's fort or castle while a huge crater is formed in the ground. (At this point, you will want to have some beer out and be hooting and hollering).

Step 7: If the enemy continues to resist: rinse and repeat until they are tired of having several tons of rock come crashing down on them.

For this to be practical you need a civilization that has not perfected advanced metalworking but has been fooling around with explosives for a while. They have ready access to lots of manpower and are not afraid of engineering challenges. They also will set up and keep a siege of an enemy position for a long time if necessary. I imagine that if the Roman Empire had stuck around and had started tinkering with gunpowder a lot, this kind of thing might occur to them.

Technically, it's an "earth cannon" with a stone projectile, but since the stone is actually keeping the charge compressed, it's a debatable point.

Some advantages; Unlike some very early heavy siege cannons, you only need to carry the gunpowder and some shovels and stone working tools, you don't need to drag 50 tons of bronze or iron cannon with you to the enemy castle. Depending on geography, you should be able to find suitable boulders close to whatever you are attacking. Earth is obviously always available. Even if the boulder breaks up, it will probably be flying in more or less the correct direction. A scattershot of boulder hunks will still be very bad for the enemy.

Answer 7

How many shots you want to fire?

One? not a too big problem. More? Umm, do you like Russian Roulette?

There was a not so historically accurate story about Tihany's (Tihany is located in Hungary) wooden cannon. TL; Dr: It fired the ball, the cannon got destroyed, but it fired the cannonball.

The problem:

STONE IS BRITTLE The problem is not with hardness, but with brittleness, less brittle materials deform gradually, brittle materials break.

My proposed solution

• I can't say too much, but I'd go with Tuffs, they can seal heat well, cheap, abundant, light, and soft.
• You can also consider using harder materials, which's recoil could be lessened by a tuff casing.

But again, you'd better ask the others as I'm not 100 percent sure about these.

Answer 8

The big question here is: "Would it still be usable after firing?"

And the answer to that most likely is no.

Stone, as a molecule has a crystalline structure like glass and sand. Glass can be really strong, like stone but it will always be brittle meaning it can't resist vibrations.

Some glass (and stone, concrete, etc.) can hold-up tremendous force without breaking, it can withstand a lot of pressure, but impulse is a different kind of force, impulse is the force that a cannon must withstand

The tool shown here is a hammer drill, it works by providing a great impulsive force through hard vibrations that will tear through stone or concrete with ease, however, this tool would never in a million years tear through iron or steel.

In summary, a stone cannon would look great and it would work for a few shots, but once the pieces start chipping away, it is going to fall apart.

Final note: This is true for all stones and crystalline structures, even diamond! A diamond cannon would also have the same problem, just because it can't be scratched doesn't mean it can't be shattered.

Answer 9

You might have better luck with a dirt/stone cannon. the main problem with the stone is how brittle it is. this would lead it to shatter after only a few shots if that. if you packed the inside of your stone cannon with a very compact layer of dirt, or other dense padding, you might be able to re-use the cannon by refilling the cushion after every shot. the only downside is that you sacrifice a good deal of the force to do this, so your cannon will really only be effective against infantry, not fortifications.