Military history is a see-saw between fixed defensive technology and mobile offensive technology. Sometimes offensive technology leaps ahead, as at the close of the middle ages when improved cannons suddenly rendered every castle in Europe obsolete. At other time defenses gain the advantage, such as on the Western Front of the Great War, when armies became deadlocked for years in their trenches. Reading Thucydides, there can be no doubt that in the era of the Peloponnesian War defensive technology was ascendant. A fourth century Greek army had little chance of storming the stone walls of a fortified city, even with a significant advantage in numbers. The main land force of the Peloponnesian League, with the best infantry in the world, was afraid to directly attack the Long Walls of Athens. We read of lesser cities withstanding sieges for months or years, before finally yielding through starvation or treachery. In the years leading up to the war, a city deciding to build fortifications was considered a reasonable pretense for declaring war, to preemptively attack before the walls made them impregnable.
Thucydides alludes several times to “engines” used to attack walls, but these seem to have been simple battering rams which had to be pushed right up to the wall to work. An alert garrison was often able to destroy them before they could do damage. During the siege of Platea, for example, the defenders were able to break apart a Spartan ram with lassos and logs. Other siege engineering techniques were even cruder, such as piling earthen ramps against the walls or trying to light fires.
In Book IV, however, we read about a new device: the first recorded flamethrower. First used by the Boethians at the siege of Delium, this device could incinerate wooden walls and gates and even crack stones. Like the first tanks deployed at the Somme, this device signaled that the balance between defense and offense was about to shift again.
Thucydides gives a fairly detailed description of the Boetian flamethrower,
Meanwhile the Boeotians … marched against Delium, and attacked the fort, and after divers efforts finally succeeded in taking it by an engine of the following description. They sawed in two and scooped out a great beam from end to end, and fitting it nicely together again like a pipe, hung by chains a cauldron at one extremity, with which communicated an iron tube projecting from the beam, which was itself in great part plated with iron. This they brought up from a distance upon carts to the part of the wall principally composed of vines and timber, and when it was near, inserted huge bellows into their end of the beam and blew with them. The blast passing closely confined into the cauldron, which was filled with lighted coals, sulphur and pitch, made a great blaze, and set fire to the wall, which soon became untenable for its defenders, who left it and fled; and in this way the fort was taken. Of the garrison some were killed and two hundred made prisoners; most of the rest got on board their ships and returned home.
For all we know, he may also have included a drawing. If so, though, it has long since been lost to history. A quick search on the web yields many modern day artists’ conceptions, but I couldn’t resist sketching my own.There is debate about what substance was burned in the “cauldron” at the end. A number of flammable substances were available in the ancient world, including naphtha, animal fat, and various vegetable oils. I’m inclined to believe that actual pitch–a simple wood tar–was used. Wood tar is easy to make and was already used by the Greeks to waterproof their ships, buckets, ropes, and other items. The besieging army might well have brought a supply with them for general maintenance. Wood tar is sticky and flammable. While it is possible to put it out with water, it takes a great deal of water to do it. Furthermore, since it is only mildly water soluble, water tends to spread the burning pitch, which makes it even harder to put out.
Sulfur from several locations around the Mediterranean and was a common household item for the Greeks, used to fumigate buildings and as a component of medical preparations. Sulfur has particular characteristics which make it useful in incendiaries: it lights easily and fast, yet burns long enough to light other things. The first matches were chips of wood soaked in sulfur that could be lit with a flint and steel then used to light fires. Modern military manuals still include several sulfur based recipes for “igniters” that can be used to set off hard to light compounds. The sulfur added to the pitch in the Boethian flamethrower would have helped insure that most of the pitch caught fire when the air blast hit it, instead of just being squirted out of the cauldron. Burning sulfur also produces nasty fumes which would have further hampered fire fighting efforts.
How well would this have worked? Lets assume that the “kettle” held about five gallons of burning pitch, about the size of a large paint bucket. Wood tar contains several chemical compounds but the most common and representative is a ketone with the formula C19H22N2O2. Based on its entry in the database PubChem, five gallons of it would mass about 43.3 lbs. The Boethians didn’t know modern chemical stoichiometry (for that matter, it has been a decade and a half since I took a chemistry class) but lets assume that, through trial and error, they made the bellows big enough to fully combust the pitch in one puff. The net combustion equation for C19H22N2O2 is probably something like:
C19H22N2O2 + O2 –> CO2 + N2 + H2O + HEAT
It would therefore take 0.07 lbs of Oxygen to fully combust the pitch. This would require a bellows that held 34.3 gallons of air at one atmosphere, plus as much more as would be required to oxidize the sulfur. A 50 gallon bellows would probably be safe, erring on the side of extra oxygen. How much heat would be released by this setup?
The combustion properties of wood products have long been of some interest to the US Forest Service, and they have released a number of pamphlets on the subject. Using the heat of combustion of pine tar from one of the tables from this study, 12,195 BTU/lb I calculate that each puff would release about 528,044 BTUs of heat energy on the enemy’s walls. Not too shabby.
The flash point of dry oak is about 900degF and its specific heat is 0.48 BTU/lb*degF. Assuming a paltry 20% efficiency, this would be enough heat in one shot to instantly ignite 244 lbs of wood, and the flames would spread rapidly. While this is scarcely as destructive as a modern napalm bomb, it is still probably no accident that wooden fortifications fell out of favor rather rapidly around this time.