For packing or insulating material a variety of substances may be used.
Asbestos and mineral wool are the best, and have the additional
advantage that they cannot burn. Ground cork (used in packing Malaga
grapes), hay, excelsior, Spanish moss, wool, and crumpled paper may also
be used satisfactorily. Of these materials crumpled paper is probably
the best, as it is clean and odourless and, if properly packed, will
hold the heat better than the others. It is wise to line the box with
one thickness of heavy paper or with several thicknesses of newspaper,
to make it as air-tight as possible. Asbestos sheeting may be used
instead. To pack the container with paper, crush single sheets of
newspaper between the hands and pack a layer at least four inches deep
over the bottom of the outside container, pounding it in with a heavy
stick of wood.
Place the inside container for the cooking kettle or the lining for the
inside of the nest in the centre of this layer, and pack more crushed
paper about it as solidly as possible. The method of packing with paper
is shown in Figure 40. If other material is used it should be packed in
a similar way.
Where an extra source of heat is to be used, it is much safer to use
some non-inflammable material such as asbestos or mineral wool. A cheap
substitute and one which is easily obtained are the small cinders sifted
from coal ashes, preferably those from soft coal. However, the cinders
from hard coal burned in the kitchen range will do. If a fire-proof
packing material is not used, a heavy pad of asbestos should be placed
at the bottom of the metal lining, and a sheet or two of this paper
should be placed between the lining of the nest and the packing
material. Whatever is used should come to the top of the inside
container, and the box should be filled to within about four inches of
the top.
[Illustration: _Fig._ 40.--Fireless cooker, showing method of packing
with paper]