What is the ampacity of 14 gauge silver wire?
In general the current rating of a wire is limited by the wire's ability to dissipate heat to its surroundings without getting too hot. That in turn depends on the maximum safe wire temperature and the thermal resistance to ambient. Unfortunately figuring out exactly what the thermal resistance to ambient is for a given installation method is nontrivial.

Unfortunately standards don't normally say how they came up with the tabulated figures but one can assume they were determined from experimental data.

We can come up with a figure for a silver wire by assuming that the acceptable heat dissipation for a silver wire of a given size is the same as that for a copper wire of the same size. We divide the resistivity of silver by that of copper and then take the square root (remember \$P=I^2R\$). 

Using numbers from this table tells us that a silver wire should have about 1.03 times the current carrying capacity of a copper wire of the same size installed under the same conditions.

This is small enough that other factors (exact installation method, material purity, how the material was worked into a wire etc) are likely to outweigh the difference between silver and copper.