Monday, February 25, 2013

Constructing a British Army Improvised Cooker

     In an earlier post I addressed the basics of improvised cooking equipment in the British Army. I find it to be a fascinating aspect of field cookery, and decided to recreate a working example. Most manuals were terribly lacking in details, but I found a good resource for construction techniques in the British Home Office’s “Civil Defence Manual of Basic Training, Volume 1, Welfare Section”, published in 1952. In the aftermath of nuclear, biological and chemical warfare and the resultant destruction of infrastructure and disruption of essential services, it was expected that large segments of the population would be in need of basic necessities.  The Home Office turned to the British Army for a solution to the problem of mass feeding.
     The Civil Defence Manual of Basic Training stated that “The improvised equipment illustrated has been successfully used for many years by the British Army and all of it can be made, at very little cost, from scrap material.” I found this to be absolutely true. The cooker was constructed entirely from scrap material that I had on hand, with the exception of two cans of high-heat enamel spray paint. High-heat enamel may not be historically correct, but I want to extend the service life of my cooker for as long as possible.
     Very little was required in the way of tools, as all of the tasks could be accomplished with simple hand tools. The only power tools I used were a drill for the oven handle (4 holes) and a jigsaw to expedite cutting of the sheet metal metal-cutting shears worked fine, but were much slower). Pug was mixed with a hoe and cement tub. Steel bars were cut with a hacksaw. British Army manuals recommended “running over repeatedly with a lorry” to flatten the corrugated metal sheets. Not having an army lorry readily available, I flattened the corrugated sheets by pounding repeatedly with a big hammer (very time consuming and admittedly not a lot of fun, but it provided great stress relief).

    My example is a combination cooker, but without the water boiler. Alternately, it could be considered an “oven with frying plate”, but with a longer frying plate. It has the following features for cooking:
1. A flat area to place pots or pans for boiling or frying, known as the “boiling/frying plate”.
2. An oven made from a metal drum.
3. A covered hot plate or hot cupboard for keeping cooked foods warm.
4. A hole in the boiling/frying plate to accommodate a stock pot, as would be done with a camp kettle. British Army 3-gallon camp kettles had tapered sides and would not fall through the hole. I just cut the hole slightly smaller than the pot. Pug was placed around the camp kettles or pots for insulation. 
     There were many adaptations of the given designs and, if I understand the instructions of the manuals correctly, the designs do not have to be adhered to unquestioningly. They could be modified to accommodate local requirements and available resources. My intent was to demonstrate the technique for building improvised field cookers. As expected, I encountered many issues not sufficiently described in manuals. These details could only be worked out by actually working my way through the construction. 
     One aid to determining many of the dimensions was in counting the bricks in photos of improvised field cookers. British Imperial bricks were most likely used, and they fit in quite well with the given measurements.
 However, I had to modify the dimensions slightly to take into account the smaller size of US bricks:
Imperial bricks (excluding joints) - 8-5/8 x 4-1/8 x 2-5/8 inches (L x W x H)
U.S bricks (excluding joints) - 8 x 4 x 2¼ inches (L x W x H)
So for example, instead of using 6 courses of bricks to achieve the proper height of the boiling/frying plate, I had to use 7. To further complicate the process, most of my bricks were locally produced and often quite old, so there were some additional minor variances in the sizes. 

When building a cooker, there are several important design considerations which should be followed to insure proper functioning:
1. The ground should be reasonably level and firm.
2. The firebox should not be more than 10 inches wide by 24 inches long and 9 inches high.
3. The firebox uses grating, grid, or perforated sheet metal, not more than 3/8 inch thick.
4. The chimney hole is centered over the top of the barrel oven.
5. The height of the chimney (from the top of the chimney to ground level) should not be less than the horizontal length of the flue.
6. Boiling/frying plate is a piece of sheet metal, not more than 3/8 inch thick.
7. Boiling or frying plate sections, kettle trenches are approximately 24 inches wide. 
8. The oven is made from a 55 gallon drum or other metal drum, preferably food-safe. It is recommended to burn out the inside of the barrel before use. Do not use a barrel that contained toxic substances. Contrary to the techniques described in the Home Office Civil Defence manual, it is not recommended to use galvanized trash cans for ovens.

Here are a few helpful hints for construction: 
1. Soak the bricks in water before laying. This prevents the bricks from absorbing too much water from the pug and possibly weakening the joint.
2. Pug is sifted earth, preferably clay-type soil, mixed with enough water until it is the consistency of thick oatmeal. Chopped dry grass or straw should be added as a binder. The amount of straw added will vary with the soil type.
3. Estimate the amount of soil that you will need, and then double that. I used about 10 cubic feet of sifted soil for my cooker, much more than my original estimate.
4. Use gloves whenever you are cutting, bending or handling sheet metal. The cut edge of sheet metal can result in a really nasty cut. 

Should you wish to indulge in constructing your own improvised cooker, you are responsible for your safety, fire prevention and compliance with local laws. Any recreation of the techniques given here is taken at your own risk. 

And now that we have the obligatory legal disclaimer out of the way, let's see how it was done.
I leveled the area, tamped the soil, measured and set out marking stakes. Then I laid out the first course of bricks (dry) to get a more exact idea of the layout. 
Note the materials laid out next to the construction area: bricks, sheet metal, metal rods, sifted soil (in the black nursery pots), chopped pine straw (in the galvanized trash can), and a 55 gallon drum.

A layer of pug is spread on the ground. Note that the near side is twice as wide as the rest. This is to accommodate the inner wall which will be the support for the closed end of the drum oven. Fortunately, for the pug I have a nearly endless supply of sticky of Alabama red clay soil and pine straw, which doesn’t deteriorate as quickly as dried grass or wheat straw.

Lay out the first course of bricks, putting about a 3/8 inch (1 cm) layer of pug in between each brick. Note that the bricks on the open end (on the right) are half bricks. The bricks are “keyed”: they are laid so that each course has the joint between two bricks centered over the middle of the bricks on the course below it.
The bottom course for the inner wall is also laid.
Put a 3/8 inch layer of pug on top of the first course and continue to lay each successive course.

After four courses are laid, the firebox grate is added, and the 5th course of bricks over it.

Close up detail of the supports for the firebox. Optionally, a metal grid or perforated sheet metal can be placed on top of the 4th course bricks, without the support rods. By using support rods, I can easily replace the firebox grid once it deteriorates through use. 
Continue until 7 courses have been laid. Build up an inner wall as shown in the photo above, to the height of the frying/boiling plate section. This is to support the closed end of the drum oven.

View from the front of the cooker.
Note how the grates are laid over the support rods to create a firebox, and the inner support wall to the left rear.

Another view of the cooker after the 7th course has been laid.
Before continuing, the base of the flue is filled in. Rubble was normally used, but I had a large amount of coarse gravel available. I laid a short retaining wall at the rear of the firebox to keep the flue material in place.

The flue slopes upward from the front of the cooker to the rear.
(Before filling in the flue, I laid down a sheet of permeable landscape fabric. This fabric is not necessary, but it serves as a barrier to insects (fire ants are a problem here) and to make clean up easier if the cooker is disassembled or moved.)

A layer of pug or sheet metal is placed over the flue base. 
As I will be using a rather thin sheet of corrugated steel for the boiling/frying plate, I needed to add some steel rods for support. If a thick sheet of steel is not used, reinforcement must also be added for the front wall of the oven section which spans the flue. Those are the two long rods on the left side. I spaced the short rods evenly, except for the large gap towards the left. The opening for a stock pot will occupy this space.

The top of the seventh course is the height of the boiling/frying plate, which is laid over a layer of pug on top of the 7th course. A hole has been cut for the stock pot. Originally, this would have been an oval opening to accomodate a camp kettle. The bricks on top of the plate are temporary, to hold the plate in position.

Next we turn our attention to the oven section. Lay several bricks for the next course of the forward oven wall and on the ends of the inner wall, to prevent the drum from rolling. A thick layer of pug is put down where the drum will rest and formed to the curve of the drum. 

Detail of the rear (closed) end of the drum. The inner wall is used for supporting the rear end of the drum. Continue to lay bricks around the barrel, making sure that the courses are keyed. Use broken or cut pieces of brick to insure a snug fit around the front of the barrel.

Ten additional courses of bricks have been laid to enclose the drum oven. Pug is used to seal around the curve of the barrel. Before the last course was laid, reinforcement was added over the top of the drum oven for the bricks that spanned it in the last (10th) course.

Two concrete reinforcing rods are set in a layer of pug on top of the top brick course. They will help to support the weight of the pug on top of the sheet metal.

A piece of sheet metal is placed over the top of the oven, and temporarily held in place with bricks.
Note the hole cut for the chimney, centered over the drum.

 Close-up detail of the chimney hole.
The material in the hole was cut and bent upward to form flanges to help secure the chimney.

 I greased the bottom half of a stock pot with cooking oil to make removal and cleanup easier. The pot was set in place over the hole in the boiling/frying plate. A layer of pug about 3½ to 4 inches (9-10 cm) deep was formed around the pot.
The sides of the frying area are then constructed from pug.
I used a rather stiff mixture of pug for the boiling/frying area. 
The stockpot has been temporarily removed to show the hole in the boiling/frying plate.
The chimney is made out of 4 inch (10 cm) diameter cans.
Both ends are cut off and one end is crimped with needle-nose pliers. The crimped end is inserted into the non-crimped end of the next can, and carefully
tapped in further with a rubber mallet until it is secure. Do not use excessive force, or you will bend the cans. If done properly, a sufficient length of stovepipe can be fabricated that is both straight and secure.
Formed out of sheet metal or a square tin, a hot cupboard was used as a food warmer. My example is constructed out of sheet aluminum. It will be mounted on top of the oven and covered with pug. 
The hot cupboard was secured in place and covered with pug. A section of chimney was put on the flange and a support built up around it. The support could be a column of bricks or pug. I cut the bottom off of a plastic nursery pot which was then, inverted, centered it over the stovepipe and filled with pug.

 View from the front.
I sized the frying plate and hot cupboard to accommodate a half sheet pan (approx. 13 x 18 inches).

The completed combination cooker. 
The handle for the oven lid was fabricated from a piece of flat steel that I cut to size, forged into shape and drilled for mounting bolts. The barrel has a clamp to hold the lid. If you are using a lid without a clamp, latches will have to be added to hold the lid in place.

The cooker with the chimney removed and the oven opened. The oven rack is a bit small, but will suffice for now. I plan on making a larger rack that will set higher up in the oven and provide more space.
View from the rear of the cooker. Although the firebox opening faces the direction of prevailing winds for this area (west), today the wind refused to cooperate and blew in from the east. I’ll have to do another test firing later in the week, especially to determine if the 4-inch diameter chimney is sufficient or whether I may need to increase the size.

Manual of Military Cooking and Dietary, 1924. H.M. Stationery Office, 1924
Manual of Military Cooking and Dietary, Part I - General. H.M. Stationery Office, 1940
Manual of Army Catering Services 1945, Part III. The War Office, 1945
Manual of Army Catering Services 1954. The War Office, 1954
Civil Defence Manual of Basic Training Volume 1 Welfare Section  (Improvisation of Large Scale Cooking Equipment), Pamphlet no. 2B. H.M. Stationery Office, 1952

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