Copper-Bottomed

For the most part, coal before the second half of the eighteenth century was quarried. It was largely dug out of pits near the surface. The earliest coalmines in Yorkshire were only 300 feet (100 m) deep, and much deeper coal were not able to be extracted. By 1770, 6.5 million tons of coal was mined annually. Forty-five years later, this had risen to 16 million tons. This increase was partly due to the use of wooden pit props to work deeper mines, initially on Tyneside, which went down to 1,000 feet (300 m). (1)

I've been reading a Brief History of Britain. Alongside the larger scale and more well known events, like the increase in the mining of coal, there are little snippets almost unseen, which I had no knowledge of at all. I love the small fact. There are times when big pictures are useful, but a small fact can tell its own unique and interesting story:

"The copper from South Wales was used, among other things, to plate the bottoms of naval vessels. As barnacles did not grow on copper, this gave British battleships a maneuverability that was invaluable, as demonstrated at the Battle of Trafalgar" (1)

My friend Julie tells me that is the origin of the phase "copper-bottomed", which of course, nowadays has a figurative meaning of genuine; trustworthy.

The Oxford English Dictionary tells me that as the original phrase - having the bottom covered or sheathed with copper, it was "used especially of  of ships, as a protection against the destruction of the planks by the teredo, and the accumulation on the surface of shells and weeds which retard the ship's motion. First applied to ships of the British navy in 1761."(2)

The Phrase Finder tells us even more

The process was first used on ships of the British Navy in 1761 to defend their wooden planking against attack by Teredo worms a.k.a. Shipworms (actually a type of bivalve clam) and to reduce infestations by barnacles. The method was successful in protecting ships' timbers and in increasing speed and maneuverability and soon became widely used. This piece from The London Magazine, March 1781, records the introduction of its use on all the ships of the Royal Navy: "Admiral Keppel made a remark upon copper bottomed ships. He said they gave additional strength to the navy and he reproached Lord Sandwich with having refused to sheath only a few ships with copper at his request, when he had since ordered the whole navy to be sheathed."

John Montagu, 4th Earl of Sandwich, may have been otherwise occupied. He is said to have once spent twenty-four hours at the gaming-table without refreshment other than some cold beef placed between slices of toast - hence giving name to the sandwich. Before long, 'copper-bottomed' began to be used figuratively to refer to anything that was certain and trustworthy. Washington Irving, in his work Salmagundi, 1807, included this line: "The copper-bottomed angel at Messrs. Paff's in Broadway." (3)

But as it points out, it wasn't all plain sailing. A combination of copper and iron is not necessarily a good one:

Copper and iron, when immersed in a suitable electrolytic fluid, like fruit juice or, at a pinch, seawater, form an electrochemical couple and the arrangement becomes a serviceable galvanic battery. Over time, the iron is eaten away to nothing by the electrochemical action.

That wasn't good news for mariners who fixed their boat's copper plates using iron nails - the iron eroded and the plates went to visit Davy Jones. Copper nails were the answer and soon afterwards ships began to be described not only as copper-bottomed but also copper-fastened. Such technically top-of-the-range ships were well thought of; an example is found in the 9th July 1796 edition of The Hull Advertiser: "She is copper-fastened and copper-bottomed, and a remarkable fine ship." (3)

Here are some examples of the phrase "copper bottomed" over the centuries:

1795   Hull Advertiser 23 May 2/1   The copper-bottomed ship Ann.

1796   G. Pearson in Philos. Trans. (Royal Soc.) 86 451   This effect of copper upon the iron bolts and nails, in copper-bottomed ships.

1814   Salmagundi (ed. 2) I. x. 246   The copper-bottomed angel at messrs. Paff's in Broadway.

1829   F. Marryat Naval Officer II. ix. 290   The wreck proved to be a copper-bottomed schooner.

1890   J. S. Farmer & W. E. Henley Slang I. s.v. A 1,   In mercantile circles, the expression has become popularly current, in a figurative sense, to signify the highest commercial credit; and first-class; first-rate. The form varies, being rendered bylaw 1 copper-bottomed, [etc.].

1894   R. L. Stevenson & L. Osbourne Ebb-tide ii. vii,   The real, first-rate, copper-bottomed aristocrat.

1937   A. Calder-Marshall in C. Day Lewis Mind in Chains 73   The attitude can then be adopted: '‥We are moderates. Good, solid, honest to God, copper-bottomed capitalists.'

1961   Times 9 Nov. 16/2   The genuine copper-bottomed novel-reader's novel is becoming a rare article.

There is also a gardening connection. And here my friend Jeff Hathaway brings the story up to date with copper paint - and a neat trick to get rid of slugs from the garden or allotment:

Barnacles along with slugs and snails, are molluscs. Slug pellets are blue because they contain copper sulphate. Now, if you don't' want to protect your container plants with unsightly pellets and risk poisoning other creatures, old copper water pipe laid around the rim of a container will keep them off off.

As you quite rightly point out, hulls of old ships were clad in copper to prevent barnacles (and other molluscs) from colonising them not just because they slow ships down, but some actually will bore into wood. Shipworm is not a worm at all but a mollusc closely related to clams. Today, most paints for ship and boat hulls also contain copper - a much cheaper option that actual copper cladding and just as effective. (4)

Links
(1) A Brief History of Britain 1660-1851, William Gibson
(2) Oxford English Dictionary
(3) http://www.phrases.org.uk/meanings/copper-bottomed.html
(4) Email from Jeff Hathaway