TIN-PLATE AND TERNE-PLATE. is a sheet of iron or steel which has been thinly coated with tin by being dipped in a molten bath of that metal. Terne-plate is a sheet of iron or steel coated with a lead-tin alloy containing about 85% lead and 15% tin. The word terve was applied to this plate be cause of its dull appearance due to the large proportion of lead in the coating. The purpose of the tin in terne-plate coating is to act as a binder between the lead and the iron, which do not alloy.
There are two processes for tinning. In the "palm-oil" process, which is the older, the plates, after being properly annealed, are scoured with sand and water and pickled in dilute sulphuric acid alternately until they are perfectly clean and bright. They are then washed in water, and after being boiled in palm oil to remove all traces of acid and water are dipped into a bath of molten tin, covered with oil to prevent oxidation. They are then taken to a second bath containing purer tin than the first. After this they are scoured with a hempen rubber and dipped in a third bath con taining the purest tin of all; then they are passed through rolls to finish the surface and regulate the thickness of the coating. In the "acid process" only a single bath of tin is required. The molten metal is covered with a layer of muriate of zinc, which acts as the flux, and by means of rolls the plates are passed through this down into the tin, to be brought out at another point in the bath where there is a layer of oil on the surface.
The process of coating terne-plate is very similar to the gal vanizing (q.v.) process. The sheets, after having been pickled, are placed in the same kind of an acid solution, except the acid is somewhat weaker. The sheets first pass into a neutralized solu tion of zinc chloride. This flux enables the lead-tin mixture to adhere better to the base metal. Burners located under the intake side of the "kettle" keep its temperature at about 700 to 725° F. The metal passes through the lead-tin alloy, under the flux, at the bottom of the kettle. As the sheets emerge through the exit rolls, they pass through palm oil. The rolls serve to pull the sheets through, and, together with the oil, they wipe off the excess metal and produce a smooth coating. This entire process is now com pleted by one machine ; formerly it was necessary to dip and redip the sheets by hand. Tongs are used to push the sheets down
through the flux box until they are gripped and pulled out by the rolls. As the sheets leave the oil bath, an air spray cools them. They then pass through a set of flannel disk rolls that squeeze off the excess oil, and on to a conveyor. They pass from the con veyor to cleaning machines, consisting of flannel disk rolls and brushes, using sawdust, peanut hulls, meal or bran further to ab sorb and to clean off the oil. The weight of coating is more diffi cult to control than it is in galvanizing. It is affected by the heat of the bath, speed of the machine, the kind of rolls used and the surface of the sheet coated. The temperature limits of the opera tion are very narrow.
The sheets employed in the manufacture of tin-plate and terne plate are known as "black plates." Formerly iron alone was used, and was of two grades, coke-iron and charcoal-iron; the latter being the better, received a heavier coating of tin, and this circumstance is the origin of the terms "coke-plates" and "charcoal plates" by which the quality of tin-plate is still designated. Later steel was used but it was found that iron plate was more durable. Now both iron and steel plates are available. (See SHLETS, IRON AND STEEL.) It has been attempted to lower the price of terne plate by using coatings entirely too light. Although much of this light-coated terne-plate is still used, high grade plate, properly coated, is obtainable. The service pure iron renders will, no doubt, cause more and more of this product to be used as the base for terne roofing plate as well as long ternes. Two different types of finishes are obtainable; "bright dry" or "dark oil." These dif ferent effects are imparted as it travels through the machine.
Long ternes are similar to terne-plates except that the sheets are larger, and the tin content usually runs about 121%. The lead in the lead-tin alloy makes long terne-plate ideal for fabrication into caskets, automobile gasolene tanks and other products in the manufacture of which deep drawing and stamping (see PRESSED METAL) is necessary. The lead serves as a lubricant in forming operations, causing the sheets to flow uniformly under the dies.