From a page on the modern oyster trade
http://www.fao.org/wairdocs/tan/x5954e/x5954e01.htm

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Transport and storage of live oysters

Oysters can survive out of water for weeks if carefully handled and kept
moist and cool but, since they rapidly lose liquor from within,
particularly if the edge of the shell is damaged, they should reach the
inland wholesaler within 3 days of harvesting to be in prime condition.
They should be carried and stored with the cupped half of the shell
downwards, and kept moist by covering with a damp cloth. They should be
packed in a manner that protects them from mechanical damage, and should be
kept at all times at a temperature between 1 and 10°C. Commercial packaging
ranges from a simple barrel, box or sack to a specially designed container
with separate compartments for individual oysters, the degree of
sophistication depending on the value of the product, the journey time, and
the market for which it is destined.
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So the old barrels would have kept the oysters edible for some time.
Oysters were far from a luxury product in former centuries:
'It's a wery remarkable circumstance, sir,' said Sam, 'that poverty and
oysters always seem to go together.' -- Charles Dickens, Pickwick
Papers,1836, chapter 22

Oysters are also smoked, a process described lower on the same page.

And they are hot pickled by being cooked with with vinegar, salt and
spices, sometimes also with sugar, all of which would preserve them. After
the early 19th century invention of bottling, if this was done properly
they would have kept for months.

RH

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