What the speaker is saying here is that being moved farther apart does not make him and his love any less connected. He is likening their love to gold. He is saying that both of them can be spread out into a very, very thin layer but that does not ruin them. Instead, it actually expands them (he says) -- it makes them cover more of an area.
So this is another way in which the speaker illustrates the main idea he is trying to make in the poem -- that physical separation cannot diminsh their love.
The subject of the simile about metallurgy begun in line 17 is the refining of gold, in which all dross and impurities are removed and only the purest and most valuable gold remains. The speaker also refers to the malleability of gold to suggest that even when lovers are apart they are still united, just like a sheet of delicate gold foil (line 24) that stretches between the loved ones so that they are still and always connected to each other. This metaphor supports the conviction that this love is deep and lasting; not merely valuable as some readers who have not digested all of the previous stanzas will reply.