Dead Metaphor

I stumbled across the concept of "dead" metaphors a while ago, and it has fascinated me ever since. My hunch is that our language is heavily littered with these terms (littered being an example) to the extent that we simply don't notice them.
The most commonly accepted definition of a dead metaphor is that the source from which the term derives its meaning has become disassociated from the current meaning of the term.

The phrase has been challenged in recent years by linguists, and I accept it is not precise - metaphors can retain their effectiveness even when remote from their original meaning, and therefore retain their claim to the status of metaphor. In fact, metaphors are often not truly dead, just dying. Arguably they start dying from the moment they are coined, and it is only the speed with which their inbuilt meaning decays that will vary. A metaphor tied to a specific cultural event or icon will die more quickly than something more universal. To illustrate this, the word "jumbo" originally drew its meaning form the elephant of the same name - see http://en.wikipedia.org/wiki/Jumbo. To most people the term no longer conjures up the mental image of a large elephant, as it must have once done. So jumbo the metaphor died in probably less than a hundred years.

Compare this with the range of ageing but definitely still active metaphors that derive from tools - sawing, chiselling, ploughing, hammering etc. The verb "to hammer" must have been used almost immediately the noun was adopted. The term would almost certainly have been used metaphorically to describe "behaving like a hammer" as in "rain hammering down" very quickly thereafter. My guess, although I have nothing to support this, is that the noun for hammer in most languages is also used as a verb, and in a wider sense than just "using the tool". So is "rain hammering down" an example of a dead metaphor? Certainly we don't use it to make the audience picture hammers beating on a roof or pavement, but it retains its force as a way of describing heavy rain - it means more than just raining, in a way that jumbo doesn't mean more than big.

Not all tools turn into verbs, let alone metaphors - we don't "mallet" things on the head, nor ladle anything other than liquid, but we do whisk things away, spoon together, and sieve evidence for clues. Is this a matter of scale - we drive cars, instead of "car" as a verb; we fly in or by planes, but don't say "I am going to plane to Stockholm", and so on? Interestingly scale is a rich source of dead metaphors. Gargantuan derives from a 16th century story by Rabelais, and there can be few listeners/readers who think of french giants when using the term. Colossal derives (surely) from Colossus, mammoth from the woolly elephants, and so on. Terms to describe the quality of being small do not appear to attract the same type of language behaviour - we don't say "Tom Thumbian" and even Lilliputian is showing off rather than metaphorical. Brobdignagian never stuck, surprisingly.