When you detect, probably 90% of the non-trash items you find will be fasteners or small hardware of some sort.
Judging the age of a nail can tell you a lot about what you are discovering.
Most importantly, it can tell you if you've stumbled across something very old and should therefore retreat and call the Deputy State Historic Preservation Officer (see instructions in our FAQ).
Here is your fast-and-dirty thumbrule 👍:
In 1886, [only] 10 percent of the nails that were made in the United States were of the soft steel wire variety and by 1892, steel wire nails overtook iron cut nails as the main type of nails that were being produced. In 1913, wire nails were 90 percent of all nails that were produced.
2. The next level of detail can be found in this brief, excellent summary:
"The blacksmith hammered the heated rod on all four sides to make a point . . . The resulting product tapered on all four sides, one of the defining characteristics of a wrought nail. In the late 1790s, machines were invented that cut nails from sheets of plate iron in a cookie cutter manner. Unlike wrought nails, machine-cut nails taper on only two sides while the other opposing sides remain a constant thickness, that of the iron sheet from which the nails were cut.
3. The following page has a lot more information than you need if you're looking in Florida, but if you search on North America here you'll get a lot of useful information:
1772 - 1820: In North America early machine cut nails with hand-wrought heads
1805 - present: Nails made completely by machine, nail head, first appeared as sprigs and brads -1805 - continuing to the present.
First generation cut nails show cut from common sides; nail heads hammered on in a separate production step.
1810 - 1820: cut nails made cut from opposing sides; heads hammered on as a separate production step.
Before 1830: irregular and possibly off-center head (see above)
Before 1830: head was hammered to permit counter-sinking
1815 - 1830s: first machine made nail heads.
After 1830: nail heads uniform & convex on both sides
1890 nail making machinery had advanced to produce cut nails at high volume and low cost and at improved reliability
"After 1825, with the use of machines that both cut and headed nails, the shank was similar, but the head was stamped level, the top being impressed by a single blow."