Sam Takoy wrote:
Hi,

I have a copper sink in the bathroom and because it doesn't have an
overflow, the water drains very slowly. . . . ,

The lack of an overflow doesn't cause sinks to drain more slowly. Your slow
draining problem is due to something else -- possibly a partial clog
somewhere in the drain line, or maybe because the sink is not close to a
vent pipe.

RogerT wrote:
Sam Takoy wrote:
Hi,

I have a copper sink in the bathroom and because it doesn't have an
overflow, the water drains very slowly. . . . ,

The lack of an overflow doesn't cause sinks to drain more slowly. Your
slow draining problem is due to something else -- possibly a
partial clog somewhere in the drain line, or maybe because the sink
is not close to a vent pipe.

Right. The (partial) clog is somewhere down the drain. Water in the sink
hits the clog and backs up, like a dam, then proceeds at only the pace the
miniature spillway permits.

A dose of concentrated sodium hydroxide (lye) will bust up what the
miniature beavers have built.

The question of a proper vent can be answered by asking another question:
Did the sink ever drain properly?