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a cap too large for it, and believed by some to be due to defects in the capping
machinery. I do not think this has any connection with the production of these
tins. When the ends of these tins are pressed with the hand a slight up and down
movement of the cap is obtained, but there is no impression of internal pressure
as in the case of a tin commencing to blow.
When a container made of light tin plate is knocked on the rim the ends may be
forced up a little, giving this slight up and down movement characteristic of a slack cap.
A knock so slight that indentation of the tin may be hardly noticeable will sometimes
produce this condition.
When a tin which is well packed (or called by the Trade "over packed") is
processed the ends may bulge, especially if the contents were cold when the capping
took place. In this case the corrugations of the caps are likely to be strained, and
thus the ends keep their convex appearance after processing and the tin appears
to be blown. The ends can be forced back into their normal position, but a reasonable
pressure is required to overcome the resistance of the ribs or corrugations which have
been strained by the expansion of the contents. There is no up and down movement
in this case, but the caps, having been increased in size by the strain, have a convex
appearance. In this type of can it is possible to press in the caps half-way round,
so that part is slightly concave, leaving the other half convex.
With a slightly blown tin this would not be possible, as on pressure the cap would
spring in ; providing it was not too far blown.
When examining canned goods care should always be taken in passing judgment
on tins showing any movement of the ends when pressed with the hands. During
examinations carried out, whether for trade purposes or at the request of the Medical
Officer of Health, slack tins should not be passed until the Medical Officer of Health
is satisfied they are not in the early stages of blowing. On one occasion a person
interested in canned goods actually asked the question, "How long does it take for
a slack tin to develop into a blown condition?" On a further occasion I was asked
to examine 10 cases of canned fish which had been classed as slacks by the examiner.
These were found to be in an early stage of blowing, as was shown by the fact that
some of the tins, when pierced under water, gave off bubbles of gas. On enquiries
I found that approximately 40 cases of the same parcel were obviously blown and
had been classed as such by the same examiner. I assume that he called the 10 cases
slack because of the belief amongst people that a tin in the early stage of blowing
is a slack.
There is one thing which must be made quite clear, there is no connection between
a slack-capped tin and one which has commenced to blow. Both may have the same
external appearance, but whereas one is a defect of the container itself, the other is
due to micro-organisms, or to the presence of hydrogen generated by the action of
the acid contents on the tin plate.
The usual method of converting a slack into a normal can is to strike the top of
the can sharply with a rubber-faced wooden mallet, causing a permanent strain
inwards in the ribs. This is a practice which should not be encouraged, and one to
which no firm of repute would resort.
The so-called "slack cap" may be found in any shape or size of can, and the
causes of genuine " slacks " may be summarised as follows:—
(1) Capping when contents are cold. This undoubtedly accounts for a large
percentage of slacks.
(2) Overfilling.
(3) Badly ribbed or corrugated ends.
(4) The use of light tin plate.
(5) A knock on some part of the tin due to bad handling.
(6) Machinery defects. This accounts for very few, if any, "slacks."
A combination of any of these may result in producing slack-capped tins.