It is not necessary to *plate* one metal on another to get a bi-metal
sheet.  The common commercial process is "cladding", where two (or
more) sheets of metal are chemically cleaned, then heated and rolled
under high pressure.  The resulting bond can be classed as either a
weld or a high-temperature braze.  Here is a quote from an old Marks'
Handbook:

  "There are many ways of producing clad steel, but the usual method
   is to make a "sandwich" of two mild steel plates and two cladding
   plates separated by an inert material in the center.  The sandwich
   is welded around the edges to prevent oxidation on heating and is
   rolled at about 2200 F.  During rolling, welding occurs and the
   plates are reduced to the desired thickness.  The edges are sheared
   and the sandwich separates into two plates, each clad on one face.
   The thickness of the clad material may vary from 5 to 50 percent of
   the thickness of the clad plate but normally is held to 10 or 20
   percent."

Some examples of the cladding process are bi-metal strips (used
in thermometers and thermostats), and precious-metal contacts
(palladium-gold on phosphor bronze).  Hoyt metal may be another.
Thickness of the cladding would provide the clue.

Here is a way to determine that thickness:

If a sample of the material is filed (or sanded) at a very shallow
angle -- as if grinding an edge on a knife blade -- the two materials
will be exposed, and their thickness expanded by the tangent of the
bevel angle. For example, a 5.7-degree bevel will give a 10x expansion.

The exact angle is not critical; with suitable expansion, the
*relative* thicknesses can be estimated.  Caliper or micrometer
measurement of the sheet, plus a little math, will give the coating
thickness.  If the coating is more than a few thousandths of an inch,
then it is likely clad.

John Rhodes
jrhodes@pacifier.com