Jeremiah 6:27

I have set thee for a tower and a fortress - Dr. Blayney translates, I have appointed thee to make an assay among my people. The words refer to the office of an assayer of silver and gold; and the manner of assaying here intended is by the cupel , a flat broad iron ring filled with the ashes of burnt bones. To separate the alloy from the silver they add a portion of lead; and when all is fused together, and brought into a state of ebullition, the cupel absorbs the lead, and with it the dross or alloy, and the silver is left pure and motionless on the top of the cupel. The people are here represented under the notion of alloyed silver. They are full of impurities; and they are put into the hands of the prophet, the assayer, to be purified. The bellows are placed, the fire is lighted up, but all to no purpose: so intensely commixed is the alloy with the silver, that it can not be separated. The nozzle of the bellows is even melted with the intensity of the fire used to effect the refinement; and the lead is carried off by the action of the heat; and the assayer melteth in vain, for the alloy still continues in union with the metal. The assayer gives up the process, - will not institute one more expensive or tedious - pronounces the mass unfit to be coined, and denominates it reprobate silver, Jeremiah 6:30. Thus, the evil habits and dispositions of the Israelites were so ingrained that they would not yield to either the ordinary or extraordinary means of salvation. God pronounces them reprobate silver, - not sterling, - full of alloy; - having neither the image nor the superscription of the Great King either on their hearts or on their conduct. Thus he gave them up as incorrigible, and their adversaries prevailed against them. This should be a warning to other nations, and indeed to the Christian Church; for if God did not spare the natural branches, neither will he spare these.