The fusion point of the mixture should not be less than Cone 32, or 1710° Centigrade. Strong fire-clays are those coarser and harder grained, and are usually more silicious and less plastic than the mild fire-clays. Mild fire-clays are very fine-grained, plastic, and easily weathered clays. They act as the binder portion in fixing the burnt grog used in pot-clays. The raw clays should be ground very fine and separately from the burnt clays. The ground burnt should be crushed from hard and well-burnt fire-clays, and should pass a sieve of ten meshes to the linear inch. The mineralogical composition of the fire-clays for making pots is important. The presence of pyrites renders fire-clays unsuitable as pot-clays. Some indication as to the subsequent behavior of a can be obtained by submitting it to a petrographic examination, and the usual pyrochemical and physical tests carried out in testing refractory materials. In this country, Stourbridge pot-clays are chiefly used for pot-making, and so conservative are the majority of glass manufacturers that they will not use other clays, although, in the writer’s opinion, many better clays exist in Great Britain, and have now been introduced and used successfully by some firms for pot-making. Ground potsherds are selected pieces of old broken pots, cleaned from any adhering glass. These selected pieces are crushed and ground in a similar way to the burnt clay, and sieved to the same degree of fineness before use. =Plumbago= glass house pots are sometimes used. These are made from mixtures of graphite, or plumbago, and raw . They are very refractory and withstand the attack of very basic glasses, where such have to be manufactured. Pot rings are made by taking a long roll of clay about 3 in. in thickness and shaping it around a circular frame. The two ends are joined and finished smoothly, the frame took away, and the ring dried. A ring is placed in each pot. Stoppers are the lids used to close the mouth of covered pots whilst the metal is being melted. These are made in plaster case molds by pressing a bat of clay into the desired shape and releasing the outer case by turning the whole upside down upon a board and lifting off the mold. An indentation is made in the middle, forming a small hole. An iron rod can there be inserted, by which the stopper can be lifted away from the pot mouth whilst hot. Stoppers are burnt before use and are made in various sizes to fit the mouths of different pots.