RR - I hope this helps
As I understand it (please correct me if I'm wrong) - it is a bit
harder than Sterling Silver, does not polish up as bright and
solders okay with silver solders
Yes, this is true. Nickel alloys are more grey than sterling but
they polish up to a bright silver color that does not tarnish.
Solder lines, if messy, will show.
Does it (Nickel Silver) tarnish quickly like brass? What does
tarnished nickel silver look like?
No, nickel alloys don't tarnish but stay as polished.
If worn against the skin does it mark the skin? (I am aware that
some people have nickel allergies - this is not what I'm talking
I don't know about marking the skin. I would think not, unless
there is an allergy.
Can it be tumble polished?
When I tumble nickel (in steel shot), it comes out all grey and
yucky. So, no.
Can it be chemically oxidised like silver can be?
Not with liver of sulfur. However, Rio has a Midas Nickel oxidizer
that they advertise as giving a black finish in recessed areas.
What care instructions should be given for it?
Really Nickel is pretty durable and can be polished with a polishing
Please note that it will grey up with regular pickling. To get the
gunk off, pickle in about 1 C peroxide with several Tablespoons of
Sparex-type liquid pickle.
Hope this helps -