A wood fire does not burn at a uniform temperature and the temperature of any wood fire will depend upon the wood, whether or not the wood is dry, and the manner in which the fire is constructed. Wood that is not dry will spend quite a bit of time burning at a (much) lower temperature as the water in the wood boils off and escapes as steam. Once the water has been boiled off, the temperature of the fire will rise. The hottest point of the fire will be when you see glowing charcoal in the middle of the fire. Even then, how a fire has been constructed will affect the maximum temperature reached in the fire. If the fire is densely built, the flow of oxygen to the fire will be somewhat reduced and the maximum temperature consequently somewhat lower than might have been attained. If the wood is just tossed together as sticks, there may not be enough density to actually create a very hot fire. The hottest wood fires come from logs that are stacked closely together with lots of kindling and starter (think grass or paper or other, similar materials) in between the logs. A fire that is built in this manner with dry wood will burn quite rapidly and, consequently, hot. Even then, some parts of the fire will be hotter than others.
A silver necklace thrown into a well-constructed wood fire might melt but it may well not melt into a puddle. If the links of the necklace are made of thin pieces of silver, you'll have a better chance of melting it than if the links are constructed of heavy chain. In any event, a silver necklace is likely to be heavily damaged by a hot wood fire but may end up more as several pieces of partially-melted metal than a pool of molten silver.
If your character
really wants to melt the necklace into a puddle, make a bellows available to them somehow.