Solid cast-iron cannon balls were one of the first uses for cast-iron when technological improvements in the early 16th century finally allowed the production of large quantities of liquid cast-iron comparatively easily and cheaply. Prior to this cannon balls had been made of stone, each one labouriously chiselled into shape by stone-masons. Solid iron round-shot predominated on the battlefield until the mid 19th century, but from about the mid- to late-16th century, hollow explosive shells or 'bombs' also started to make an occasional appearance. Casting solid shot was comparatively straight forward, but making a hollow shell is considerably more difficult. 

Does anyone know how such hollow shells were made?


A diagram of an 18th century mortar and hollow shell.

A solid ball can be cast in a simple two-part mould. Refractory sand is packed around a wooden 'pattern' in the middle of a two part mould box. When the sand had dried the two halves are separated and the pattern removed (to be used again) leaving the cavity for the cannon ball as well as channels for pouring in iron and for air to escape.



The boxes are brought together again, securely clamped together, and then the molten metal is poured in to fill the cavity. Feeding the metal in at the side, at the junction of the two moulds, avoids damage to the interior of the mould. A separate vent at the top allows displaced air to escape and also provides a dead-head where any bits of slag or sand can collect at the surface, and it also provides a reservoir of liquid metal to compensate for the shrinkage caused by cooling and solidification of the metal.



Once the iron has cooled, the boxes are separated, the sand broken up and the metal ball removed. Flash from where the two halves of the mould joined, as well as the sprue created by the casting channel where the iron was poured in and the from the top vent, would have to be cut off with a hammer and chisel. Typical cannonballs therefore invariably show three distinct marks: the mould seam around the circumference; the filler-hole sprue; and, vertically above the plane of the mould seam, the mark of the vent hole. And of course with a suitable pattern and mould you can cast several cannonballs at the same time.


This is a wooden pattern for making six round shot, dating from, I think, the American Civil War, but a similar pattern could have been used at any time over the previous three centuries.



A hollow sphere can be made using essentially the same method but is considerably more difficult. It requires a sand ball to be supported centrally within the cavity, either on a pedestal of sand or with a bar from above, or transverse across the cavity. Again when cooled the sand is broken up and that within the centre of the metal ball broken out with a rod. It's tricky because the inner core needs to stay centrally located as the dense liquid metal flows into the mould and rises up between the inner, less dense, sand ball and the outer mould wall.



Alternatively I can think of two other methods that could have been used. 

Casting a hemi-sphere is not much different from casting a bell, which had been done for centuries, although the higher melting temperature of cast iron (typically 1200-1300°C depending on the exact composition) as compared to bronze (typical bell metal with roughly a 4:1 copper to tin ratio melts at just over 900°C) does add some complications. Two cast-iron hemi-spheres could then be joined together to make a hollow ball. But again I'm not sure how it would be easily done using 16th to 18th century technology, and it has to be done well enough so that when the resulting shell is used, the gunpowder charge within the shell doesn't explode within the gun barrel when fired.


Casting a bell at the Whitechapel bell foundry. Being specialist bell makers they have a custom-made bell-shaped mould box but the principal is the same. Note also the liquid metal is being poured in one hole while there is another on the other side to vent out the displaced air.

Another option would be to use a 'slip casting' method. Liquid iron could be poured into a spherical mould, exactly as for producing solid shot, left for a period of time while the metal starts to solidify on the outside adjacent to the mould wall, and then the remaining liquid centre poured out to leave a hollow ball. The inner surface would probably be quite rough and the walls of somewhat variable thickness, but to my mind this could be a reasonably cheap and satisfactory method. But is that how it was done?



A metallurgical examination of ancient shells would certainly give clues to their production method but I haven't found any results of such a study. The trouble is of course that complete explosive shells are very rare as archaeological artifacts. Solid cannon balls could and sometimes were reusued, to be fired back at the people who'd fired them at you (although like stone shot they might well shatter if they hit sold masonry walls). But explosive shells, by their very nature, are single use munitions and if they have functioned properly they will have been completely destroyed on their first use.

So, does anyone actually know how hollow explosive bombs were made?

Just a couple of additional points...

Some idea of the relative importance, and hence cost and ease of manufacture, of hollow shells compared to solid cannonballs can perhaps be gained from the relative numbers used.  A 1592 inventory of the English Ordnance Office stores lists 73,944 pieces of solid iron round shot, 16,784 of stone shot,  but of "hollow shot armed with fireworks" just 38. Even as late as the American Civil War (ie just over 50 years before WW1 with all that war's ghastly variety of high-explosive shells, incendiary shells, armour-piercing shells, anti-personnel fragmentation shells, and even poison gas shells) solid cast-iron round shot still accounted for about 70% of all the munitions used. The technology and expertise for casting iron obviously improved between the mid-16th and mid-19th centuries, but the solid balls fired at Gettysburg in 1863 would still have been almost indistinguishable from those fired against the Spanish Armada in 1588.

Further, just to illustrate how tricky it could be to produce hollow shot, Abraham Darby the elder, whose innovations in the early 18th century did much to boost Britain's lead in iron production and use, obtained his first and only patent (1707) for a new method of producing cast iron cooking pots. Darby's new method of casting pots was bound up with his innovative use of coke in place of charcoal - it would only work if the liquid iron he used to pour into his moulds was made with coke. Domestic round-bellied cooking pots, ubiquitous in nearly every home in the country, had usually been cast in bronze or brass, like bells (Darby had served his apprenticsehip in a Birmingham brass works and his business at Coalbrookdale started out as a brass foundry) because casting pots in iron was difficult and too expensive for producing domestic cookware. Iron pots were made on the continent, with a consequently large importation of pots into England, but they were expensive. On the continent they used charcoal but, in order to get the right structure in the thin walls of their iron pots, they had to pour their metal into moulds that were very hot, thus being obliged to use an expensive moulding process where the sand grains were bound together with clay, the so-called loam process. Darby’s patent specifically says he used no clay and his moulds were not be pre-heated. His critical innovation was the use of coke which allowed him to control the iron composition easier, thereby allowing a cheaper moulding process and without the need to pre-heat the moulds before casting.


One of Abraham Darby's cast-iron pots.

OK, admittedly a wide-mouthed/thin-walled pot is not quite the same as a thicker-walled, spherical hollow ball with a very narrow opening, but the comparison does illustrate that 150 years or so after hollow shells first appeared, making them in cast iron was still far from straight forward.

MM, did also a bit research on the net, but did't find not that much more. You as a "metal engineer" are that much better placed I think...
I found two times the idea that you mentioned of a mould with a hole with a "clay"? tap in it and filling  at another hole with liquid iron, cooling the mould very quickly and while the iron is still liquid in the middle pour that out along the clay tap, so having a remaining hollow iron ball...
I find that supposition seducing...
I read also about a kind of round sack in leather wraps with explosives and surrounded or incapsulated with brass or iron bands, which could then fragmentate during explosion...perhaps first (my supposition  ) before and together with a solid shot? 

A discussion I found and they say as you that the hollow shots with explosives only appeared after the 18th century in numbers...
http://boards.straightdope.com/sdmb/archive/index.php/t-78935.html


Kind regards from Paul.

It is still a matter of dispute (locally, at least) how much Darby gained from the experience of his cousin Dud Dudley, either directly (he is known to have visited Dudley's works) or from his book "Metallum Martis". I reckon using a chill through the core (or several chills for that matter) could stabilise your sphere's central core sufficiently to produce a spherical shell, but I do wonder if Shrapnell's ammunition required the shell production to be modified.

This is a link to an article about how hollow spheres were manufactured during the ACW:

Artillery shot





The core is formed about a hollow iron spindle, perforated with small holes through which escape the steam and gases generated by the heat of the metal. The core is centered in the mold by means of a gauge, and is supported in that position by the spindle which forms the fuze-hole. The spindle is perforated with small holes to allow the escape of steam and gas generated by the heat of the melted metal; that part of it which forms the fuze-hole is coated with sand to prevent adhesion. When the ears for the shell-hooks are cast in the projectile the necessary projections for their formation are placed in position before drying the mold. In pouring the melted iron into the mold with the ladle, care should be taken to prevent scoria and dirt from entering with it, and for this purpose the surface should be skimmed with a stick of wood. After the iron has become sufficiently hardened the flask is removed, the sprue-head is broken off, and the composition scraped from the outside of the casting. The core is then broken up and removed, and the interior surface cleaned by a scraper. The projection at the gate and other excrescencies are next chipped off and the surface of the projectile is smoothed in a rolling-barrel, or with a file or chisel if found necessary. The fuze-hole is then reamed out to the proper size and the projectile is ready for inspection.

GG and Trike, yes casting hollow balls would be facilitated by the use of hot-tops, pre-heated moulds, suitable chills, and iron spindles etc... but would these be in use at the earliest dates? Actually yes, I think they probably could. I think that in England at least (and by the mid 16th century England was the pre-eminent European country for cast iron) successfully casting of one-piece hollow shells started at about the same time as for one-piece entire cannon.

Henry VIII's army were definitely using mortars at the siege of Boulogne in 1544. This is a copy of the wall paintings of Cowdray House in West Sussex. The original paintings, which depicted the 1544 English siege of Boulogne, as well as the 1545 attempted French invasion and the naval battle in the Solent which resulted in the sinking of the Mary Rose, were completed by 1548, just a couple of years of the events themselves. Sadly the originals were destroyed by a fire in 1793 but fortuitously pen-and-ink drawings had been made a few years earlier, and these were published as watercolour paintings in 1785. 


Here at right, in a detail from one of the drawings, English artillerymen can be seen hammering fuses into round shells (supported on tall three-legged stools) in a scene that would be instantly recognised by Napoleonic gunners. Furthermore accounts of the siege describe the effects of these explosive bombs in starting fires and in creating casualties within the town. 

If these really are gunpowder-filled, cast-iron, hollow shells - and everything suggests that they were - then it is a very early date for them. 

Blast furnace technology, with its ability to get sufficiently high temperatures to produce large quantities of liquid iron, from a raw mix of iron ore and charcoal, seems to have been developed first around Namur in what is now Belgium in the previous century. However most of the product was to provide a feed stock for further refining and eventually processing into wrought-iron bars: the cast-iron itself was not seen as a final, useable product, until towards the end of the 15th century it started to be used for cannon balls. The cannon themselves were still being made almost exculsively in cast bronze (or laboriously built up from bars and hoops of wrought iron in the same way wooden casks were made - hence the term gun barrel). In principle, blast-furnace technology could also be used to cast iron cannon. But early attempts in Europe did not work. Partly, there was little incentive: iron guns were markedly heavier than bronze, so more difficult to move around and to handle in battle and in sieges. Furthermore bronze was readily available and bronze-casting furnaces did not have to reach the high temperatures required for casting iron. But England didn't have readily accessible deposits of copper for the bronze and so had to import it from the continent. It did however have iron ore as well as a long-established iron industry, centred around the Weald of Sussex/Kent, where locally mined iron ore was refined in charcoal-fired bloomeries, to produce bars of wrought iron. 

Unusually for the notoriously penny-pinching Henry VII, he does seem to have invested in trying to build up the iron industry by encouraging the development of the new blast furnace technology. Henry authorised and financed the construction of a major new ironworks at Newbridge in Sussex, including in 1491, the first recorded blast furnace in England. This was built specifically to produce cannon-balls and the simple iron fittings for gun-carriages. Henry VIII continued his father's patronage of the Wealden iron industry but even so for his first invasion of France in 1513, while the "gunstones" were mostly English-made cast-iron shot, the cannon themselves were all still cast bronze and mostly imported. The famous 'Twelve Apostles', twelve large cannons specially commissioned for the war, were all cast bronze guns manufactured at the Malines foundry near Antwerp.

But by the 1540s, all that had changed. In 1543 the master foundryman, Ralf Hogge (or Hugget) cast the first iron cannon for his unlikely employer: a Sussex vicar, William Leverett, who since 1539 had held the post of "goonstone maker" (ie gunstone maker) to the king. (Levett is not a normal English name, and he may have originally come from an immigrant family. Certainly he brought in another foreign expert to help his business, a Frenchman, Peter Baude, who had been casting bronze cannon for the King in London.

Things developed rapidly and in 1545, parson Levett was ordered to produce 120 of his state-of-the-art cannons as well as a large amount of ammunition, and by 1553 the Board of Ordnance in London had purchased more than 250 of Levett's guns. Suddenly the English iron-masters had become the yardstick by which armorers were measured. Because of the proximity of Wealden timber for charcoal, waterpower for the blast furnace bellows, the importation of foreign (primarily French) ironworkers, and effective new casting methods, the English guns were superior and cheaper than those manufactured on the European continent. The new English guns were so effective that laws were quickly passed to prevent their export to enemies on the continent. By the mid-16th century there were about 50 blast furnaces and forges, and that number had doubled 25 years later. Not all of their out put was now for military use: improvements in house design led to the building of chimneys and the need for iron fire-backs to protect the brickwork, while another popular side-line, at least locally, was in cast-iron funerary memorials in churches.

So, returning to the explosive shells depicted as being used during the 1544 siege of Boulogne ... it would seem that they may well have been cast, in one piece from cast-iron, as a co-development of the innovative manufacture of whole one-piece cast-iron cannon. Indeed they may well pre-date cast iron cannon by some years. In Charles Cruikshank's book 'Henry VIII and the invasion of France' (publ.1990), which describes in detail Henry VIII's military campaign against France in 1513, he mentions mortar-fired 'bombs' being used during the siege of Tournai (September 1513). It is not apparent from the contemporary reports exactly what these missiles were - whether they were simple incendaries or more advanced explosive devices - but they apparently had such a great effect on the city's morale, that they may well have been one-piece, cast-iron, explosive shells.

This is a 16th century German woodcut of Landsknecht artillery. The one on the left certainly looks like a mortar:

Link to an article about bronze guns:

Bronze Artillery 16th -18th centuries


Figure 1 is an illustration from an English translation of an Italian book of 1537 showing the 4 basic types of artillery, namely Culverin, Cannon, Perrier and Mortar.

Yes mortars - designed to fire at a high angle and so lob a missile over walls, rather than fire directly at them to try and break them down -  have certainly been around almost as long as there was gunpowder. But I'm never quite sure whether they were firing a solid ball (to destroy buildings hidden behind fortifications); firing some sort of burning incendary (to set buildings alight); or firing an explosive shell (to do both of the above as well as simply strike fear in the defenders).

For example here's a late 16th century mortar being used to bombard a town, but it would appear to be firing a simple incendary, such as a ball of rope and cloth doused in pitch, oil or turpentine (ie exactly the same sort of thing often lobbed over a city's walls by a trebuchet in the days before gunpowder):

A "Carcass", a type of incendiary shell, first used in 1672 according to wiki:

Carcass







Scene from the 1992 film "Last of the Mohicans", the final minute of the clip shows mortars in action:





From Encyclopedia Britannica:
Shell, variously, an artillery projectile, a cartridge case, or a shotgun cartridge. The artillery shell was in use by the 15th century, at first as a simple container for metal or stone shot, which was dispersed by the bursting of the container after leaving the gun. Explosive shells came into use in the 16th century or perhaps even earlier. These were hollow cast-iron balls filled with gunpowder and called bombs. A crude fuse was employed, consisting of a short tube, filled with a slow-burning powder, driven into a hole through the wall of the bomb. Until the 18th century such shells were used only in high-angle fire (e.g., in mortars) and confined almost entirely to land warfare. In the 19th century, shells were adopted for direct-fire artillery, notably in the form of shrapnel (q.v.).

MM, thank you very much for the message of today about the history of the cast iron cannon including the Namur story that I wasn't aware of. It can be here or at the BBC that we did a study about cannons and their firing power. In any case I made a study to make a comparison between the English and Spanish firepower on the ships and I remember that the first cannons were made of iron strips bound together with rings and the cast iron cannon was quite an innovation against the bronze and more resistant, which gave the English ships an advantage over the Spanish ones in the time of Elisabeth.
If I remember it well I mentioned also the Mary Rose and thinking now about cannon balls did some quick research if there were no hollow ones at the Mary Rose, but if I understand it well it were solid ones?
https://www.dailymail.co.uk/sciencetech/article-5481523/X-ray-probe-launched-save-cannonballs-Mary-Rose.html


Kind regards from Paul.

The Siege of Hohentwiel during the Thirty Years War.

G is notated as "Fewr Morser". The projectiles fired from this battery look to be smoking, ie fused:

That clip from "Last of the Mohicans" is interesting in that at 3:40 it clearly shows the bomb's fuse being lit immediately before the mortar itself is fired. I may well be wrong but I thought that by this date (1757) it had been realised that this rather hazardous two step ignition was unnecessary as the fuse would ignite anyway with the firing of the main charge. However this issue had been a matter of serious disagreement amongst gunners: should one put the shot into the mortar fuse-upwards or fuse-downwards? With the fuse-upwards it was thought necessary to light the fuse independently, but this two stage ignition was hazardous. If placed fuse-downwards one could rely on the mortar blast to ignite the fuse, but it was difficult to manipulate the heavy shot into the barrel with the fuse correctly oriented downwards. Furthermore it was feared, probably with good reason given the dodgey fuses of the time, that when fired with the bomb fuse-downwards, the entire charge within the bomb might prematurely ignite, causing the shot to explode before leaving the mortar — resulting in a burst mortar and a dead gunner. 

So the shot was usually placed fuse-up in the mortar and then lit immediately before the mortar was fired (as in the youtube clip). But as I say, eventually it was realised that the fuse usually ignited from the blast of the main propellant charge however the bomb was positioned.

This drawing, admittedly from a late Victorian book (W. W. Greener "The Gun and its Development", Cassell & Co. Ltd., London, 1881), illustrates the hazards of firing a mortar and bomb by the double ignition method; having lit the fuse you've got just a second or two to fire the mortar, and if it fails to fire, a not uncommon occurrence, you're stuffed.



I also finally found an image of an intact early cast iron explosive shell, as depicted in a short news article in Der Spiegel (in English).



The shell was probably fired in 1672 by the army of the Prince-Bishop of Münster, Christoph Bernhard von Galen, during the bombardment of a fortress near Diele in North-West Germany - but it fell into the castle moat and so was extinguished. The object is quite a bit bigger than a football and was cast in one piece, from iron, with walls a uniform five centimeters thick - so it seems to have been cast around a central spherical sand mass which was supported on a sand pillar, would itself form the fuse hole. The bomb still contained around seven kilograms of gunpowder and the fuse, a wooden tube reaching down into the centre of the ball, was intact. Attached to the outside of the ball was the remains of netting made of cords and the whole was coated with bitumen and wrapped in rough cloth. It is surmised that this was to ensure the ball would self-ignite from the blast when the mortar was fired.

That is a good picture of cannonballs from the Mary Rose, Paul, ... and note the distinctive marks: a thin 'flash' line around the circumference and at 90° to the plane of this, a round mark where the outlet sprue originally was (subsequently cut off). But interestingly I can't see any that show a similar round mark of the inlet sprue which would usually located on the circumferential flash line. This may be because none of the balls in the photo are facing the right way, or it could mean that there simply isn't one because the metal was poured into the same hole the displaced air came out. That would certainly make for a simpler mould but, just like trying to fill a bottle with water from a tap, even with a steady hand you might still get some splattering as the displaced air has to comes out by the same hole as the liquid is going in - which is of course rather hazardous when the liquid being poured is at a temperature of about 1400°C.

The Mary Rose is very unlikely to have ever carried any explosive shells (nor indeed were any ships-of-the-line up to at least the Napoleonic wars). In ship-to-ship action explosive shells of the type then available are not really suitable. It's one thing to try and hit the side of a ship by firing a cannon pointed directly at it, but quite another to try and drop a shell fired in a high arcing trajectory onto its deck. And if you tried to fire an explosive shell directly at a ship it would probably just bounce off. Being only partly made of iron, shells were considerably less dense than solid shot and so lacked penetrating power. Furthermore the shells of the time only had a timed fuse, not one designed to explode the charge on impact, and so, having bounced off the enemy ship's hull it would have fallen in the sea, and been simply snuffed out.

Mortars were put on ships in the 18th century, in particular onboard the vessels called 'bomb ketches'. But these ships were specifically designed for the naval bombardment of shore fortifications ... such as the famous bombardment of Fort McHenry during the war of 1812, as immortalised in words of 'The Star Spangled Banner': "And the rockets' red glare,/the bombs bursting in air,/Gave proof through the night/ that our flag was still there...". 


The naval bombardment of Fort McHenry during the wider Battle of Baltimore, 13 September 1814.

Bomb ketches typically had the masts set back away from the field of fire (hence the ketch rig), heavily reinforced hulls both to support the immense weight of the mortar but also the sudden force when it was fired, and the standing-rigging (ie the non-moving rigging that supported the masts) was usually made of iron chain, as hemp rope risked being burned by the blast of the mortar. Here's a model of the Napoleonic-era, British bomb ketch HMS Grenado - the fixed heavy mortar was set well down in the hull (for stability), and just forward of the main mast but directed slightly off the centre line (for obvious reasons).



Although the Mary Rose didn't carry any explosive shells, the inventory of 1514 does list "balls of wildfire" (just two) and "wildfire arrows" (74 complete, plus about a dozen more, in pieces, but ready to be assembled), amongst her store of munitions. We have discussed what these might be on the Tudor Wildfire thread, but I suspect they were something like an early version of the 'carcass' mentioned above by Trike -  that is a fiercely-burning sort of incendary - to be fired by a bow or just lobbed by hand at short range rather like a 'molotov cocktail', and designed principally to set the enemy's sails and rigging alight.

Thank you very much, MM,  for the explanation why the hollow cannon balls were not suitable on ships and as you put it, it is quite logical.
And then further about the "bomb ketches" about whem I never heard of.

Kind regards from Paul.

There is a certain irony (pardon the pun) in Henry VIII's encouragement of the English iron industry. Although the first documented blast furnace is the one built at Buxted in the Sussex Weald in 1491 (with the support of Henry's father), there is archaeological evidence that the Cistercian monks at Rievaulx abbey in Yorkshire may have been operating a highly efficient blast furnace decades earlier, but it and all their experience were lost when Henry dissolved the monasteries.

The Cistercian monks, as well as being expert is agriculture and building were also skilled in mining, metallurgy, engineering and the use of water power, all probably fostered by their deliberate policy of sharing technical knowledge throughout their order. Typical medieval iron 'bloomery' furnaces were only about about 1m in diameter and 2m tall, and capable on each firing of producing perhaps a kilogram of iron, but as solid pellets that needed further hammering to consolidate into a lump and to force out the solid impurities. By the mid-15th century however it seems that the Cistercians in Northern France and Southern Belgium had developed the simple bloomery furnace into a true blast furnace, several metres in diameter and, with an air-blast provided by water-powered bellows, capable of producing a ton or so of liquid iron each day. And from France/Belgium the new technology quickly spread to other Cistercian abbeys throughout Europe - including, it seems, England.

Recent archaeological investigation of a site as Laskill, which was an out station for nearby Rievaulx abbey in Yorkshire, has revealed the base of an iron smelting furnace that was 5m across, and moreover chemical anaysis of the residual slag from the furnace shows it to have a very low iron content. Similar anaylses of the slag from 16th century Wealden blast furnaces generally show a much higher iron content, and so they would have been much less efficient that the furnace operated by the monks. Indeed the slag from the furnace at Laskill is almost comparable with that produced by a modern blast furnace.

Rievaulx abbey was dissolved by Henry VIII in 1538, the monks were expelled and as was standard procedure the confiscated monastic buildings were rendered uninhabitable and stripped of valuables such as roofing lead, while the lands were granted to the Earl of Rutland, one of Henry's advisers. The abbey's "bloomsmithy" was listed amongst the effects siezed but seems never to have been operated again (and probably the knowledge of how to do so had been dispersed with the abbey's monks and lay workers). Had the monks been allowed to share their blast furnace expertise it would probably have advanced England's iron industry by many decades.

MM, thank you very much for this interesting message about the Cistercian monks and their abbeys. As the blast furnace from Namur.
They did a lot here in the plains of Flanders too, as landwinning from the sea. I discussed the reintroduction of the brick in Northern Europe after an absence of centuries since the late Roman period. If you recall it on the BBC with a doctor (general practician?) who was a collector of bricks. I think I discussed it here again on these boards. And there I found that there was a discussion among academici if it were indeed the Cistercians who reintroduced it really or that it came from the South via other channels. Will see if I find it here back.

Yes, perhaps the BBC can start a series: "What the Cistercians have done for us?"...

Kind regards from Paul.