[Illustration: ILLUSTRATION OF FOUR SCOTT PATENT DOUBLE EFFECT SALT EVAPORATORS, WITH AUTOMATIC SALT DISCHARGERS, SALT CONVEYERS, AND HYDRO-EXTRACTORS] The panning process, though carried out in the best possible manner, will not completely remove from the kettle all the separated calcium sulphate, but some of it, together with separated salt, will bake on the bottom and sides, forming an incrustation constantly increasing in thickness, though at every refilling of the kettle with fresh brine much of this adhering salt re-dissolves. This incrustation increases much more rapidly in the front kettles than in those nearer to the chimney, since, a front kettle is usually drawn every 4 or 5 hours, while a back kettle often requires from 24 to 36 hours before a sufficient amount of salt has separated. Moreover, a front kettle holds 150 gallons of brine, while those nearest the chimney contain but 100 gallons. Usually, in 5 or 6 days the incrustation becomes so thick that it interferes very materially with the evaporation, causing a great loss of fuel, as gypsum is one of the poorest conductors of heat. The workman therefore draws the salt from the kettle, removes the remaining brine to within a few gallons, and refills the kettle with fresh water. After a continuous boiling of about half an hour, the greater part of the adhering salt has dissolved and the rest of the incrustation can easily be removed. The time a salt block is in operation is between 10 and 15 days, and the manufactured salt, according to the State laws, remains undisturbed for 14 days for drainage. A salt block usually cools sufficiently in 24 hours for the kettles, grates, arches, etc., to be properly cleaned and made ready for the next run, so that about two runs can be accomplished per month. The quantity of salt produced in 24 hours in a good salt block, with average good coal dust and brine, is from 500 to 600 bushels of 56 lbs. each, and the amount obtainable by the burning of 1 ton of 2,000 lb. of this fuel varies from 45 to 50 bushels. There are two salt blocks at the Wyoming Valley, at Warsaw, in which the Onondaga kettles are heated by steam instead of direct fire. Here, in place of the brick arches in which the kettles are hung at Syracuse, they are supported by a framework, and each kettle is surrounded by a steam jacket covered with a non-conductor. Moreover, the kettle is made much thinner for the better transmission of the heat. The steam enters the jacket at the upper end of the kettle at one side, and the condensed water escapes by a valve below it, to be returned to the steam boiler. The method of manufacture of the salt does not differ in any particular from the Onondaga method.