Blue Circle Cooperages in 1924
Before the introduction of paper bags in the late 1920s, Portland cement was shipped in two types of package: jute (or sometimes cotton or hemp) sacks, and casks. Sacks were offered in a wide variety of sizes: there were 10, 11, 12, 13, 16, 20 or 25 to the imperial ton, mainly the larger sizes. Sacks were mainly for land-borne trade.
Casks were reserved for the export trade, which had once been a major part of the industry's output. Cement packed in casks was shipped all around the world, and because of the high density of cement, they could be shipped as ballast, free of charge. So casks needed to be robust and reasonably water-tight. Since there was little chance of casks being returned, new casks were used. Casks typically held 350-400 lb of cement. The US standard size was 376 lb (170.6 kg), while Germany standardised on 170 kg (374.8 kg).
Plants concentrating on exports might pack half their product in casks, around six casks per ton of cement. So a plant exporting 3000 tons per week would need 18,000 casks. Clearly, this staggering volume of casks was not available commercially, even if the transport costs of purchased casks could be borne, so casks were made on site. In a 72-hour working week, a production rate of 250 per hour was required. Staves, at 24 per cask, had to be produced at 6,000 per hour.
The Blue Circle plant schedules of 1924 (see APCM and BPCM) describe the extent of the cooperages at various plants. These employed large numbers of specialised machines for producing casks on a semi-automatic basis. These machines were considerably more sophisticated than any of the equipment used to make the cement. The survey captures a snapshot of the industry at a time when the big plants' cooperages were at their maximum development. After WWI, the use of casks went into rapid decline, and some plants had already shut down their cooperages. Decline was mainly because steel drums were making headway for waterproof packaging, and because the export market had contracted.
The 1924 survey shows ten producing plants with high-volume cooperages. Various other plants made casks in a small way, mostly manually, and some closed plants had cooperages in process of being dismantled. Casks were assembled from a set of wooden staves - usually 24, held together with steel hoops, and closed with head plates cut from joined planks. The cooperages all had a similar collection of equipment, as follows:
- Stave jointing machines: these cut planks of wood to the correct length, and routed the sides to the required convex profile, and the correct 7½° chamfer.
- Stave tongue-and-groove machines: these grooved one edge of the stave, and formed a tongue on the other.
- Stave crozing, chiming and printing machines. The croze is the slot that accommodates the edge of the head, and the chime is the tapered chamfer at the end of the stave that allows the head to be snapped into place. There were two approaches to these: either each stave could be machined individually, or the staves could be assembled into a cask, and the ends then machined in place.
- Trussing machines or bell-formers. Traditional coopering involved assembling the staves, hooping one end, then trussing the other end - typically a "choke" cable pulled tight to draw the ends of the staves together. More automated plants later used "bells" - conical castings placed over the stave ends and rammed down by hydraulic pressure. Magnetic bells could hold hoops in place and position these at the same time.
- Head turning machines: these cut the head-boards to a circle and machined the edge to fit the croze.
- Head tongue-and-groove machines: these cut planks to length, and cut tongues and grooves in their sides so that they could be assembled into an 18" square board.
- Head printing machines: these might print a logo directly on the head, or print paper labels for attachment.
- Hoop punching machines: these cut the steel strip to length, and punched rivet holes at correct pitch.
- Hoop rivetting machines: these joined the ends of the strip into a hoop.
- Hoop splaying machines: these stretched the hoop to size, with two different conical angles: casks usually had a hoop at each end, and two nearer the centre (the "bilge").
- Hydraulic pumps: these provided the power for the bells.
Hundreds of patents, with diagrams, can be found for each of these titles.
The ten operating cooperages were as follows. The links connect to the respective entries in the surveys.
| Plant | Swanscombe | Bevans | Burham | Peters & West Kent | Kent | Crown & Quarry | Martin Earles | Wouldham | Halling Manor | Johnsons |
|---|---|---|---|---|---|---|---|---|---|---|
| Capacity T/week | 6750 | 5750 | 4600 | 3000 | 4500 | 2200 | 2500 | 3500 | 2000 | 2500 |
| Est. Casks/week | 15600 | 13200 | 8200 | 6200 | 6000 | 5600 | 5600 | 4500 | 4400 | 4400 |
| Stave Jointers | 8 | 6 | 3 | 3 | 2 | 3 | 3 | 2 | 1 | 3 |
| Stave Tongue & Groovers | 9 | 6 | 3 | 3 | 2 | 3 | 3 | 2 | 1 | 3 |
| Stave Crozers | 0 | 0 | 0 | 1 | 2 | 0 | 1 | 0 | 0 | 0 |
| Stave Printers | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 |
| Stave Crozer & Printers | 3 | 2 | 2 | 0 | 0 | 1 | 0 | 0 | 1 | 1 |
| Trussers | 0 | 5 | 0 | 0 | 0 | 0 | 2 | 3 | 0 | 0 |
| Bells | 12 | 9 | 8 | 6 | 4 | 5 | 4 | 6 | 4 | 5 |
| Head Turners | 9 | 5 | 4 | 1 | 1 | 2 | 0 | 2 | 2 | 1 |
| Head Tongue & Groovers | 3 | 4 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Head Printers | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 0 |
| Hoop Punchers | 3 | 4 | 3 | 3 | 2 | 2 | 2 | 2 | 2 | 2 |
| Hoop Splayers | 6 | 7 | 4 | 3 | 2 | 2 | 3 | 2 | 2 | 1 |
| Hoop Rivetters | 10 | 7 | 4 | 4 | 2 | 3 | 3 | 2 | 2 | 3 |
| Hydraulic Pumps | 1 | 6 | 4 | 3 | 2 | 2 | 2 | 2 | 2 | 3 |
| Sprinklers | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 1 |
There is limited information about the industry outside the Blue Circle group. Since most of the coast-based exporting works were absorbed by the combine, only a few of the competition had cooperages. These included Aberthaw, Ellesmere Port, Warren and West Thurrock, the latter being by far the largest.