=77. Design of Sash.=—In designing a sash for fire-retarder frames, it is necessary, in order to comply with the Underwriters rules and regulations, to observe that no single light exceeds 24 in. × 30 in. The metallic frames are generally constructed of No. 22 galvanized steel, while the sash are made of a lighter weight, generally No. 24. In unusual localities, where the frames are likely to be subjected to the influence of gases, with known affinity for iron or galvanizing, it is permissible to make the metallic frames of 18-ounce copper, though such frames are not considered the equivalent of an iron frame as a fire-retarder, and such frames should never be used in elevator, vent shafts, or fire-retarder partitions that are liable to intense internal fires. In order to better explain the construction of the commercial frames, Fig. 36 is given, which illustrates one of the best frames in the market. In the figure, a vertical cross-section through the window-head sill and parting rail is illustrated. It will be observed that these frames can be neatly framed with architrave mold and stop, as designated at _a_ and _b_. It will also be observed that the head for the top sash is beveled, as indicated at _c_, so that a tight joint is insured by the edge of the sash coming in contact with the bevel, and thus compelling a close connection. The parting rails are also constructed with a straight piece entering on a bevel _d_, so that at this point a tight joint is also secured. By the several offsets in the sill, wind and rain stops are provided, as indicated at _e_. Sashes constructed in this manner can be made to slide freely, noiselessly, and be made tight against weather and wind, as well as being secured against annoying clattering, or rattling. The sills of metallic frames are generally filled with cement, and sometimes the heads are similarly made solid. Any unusually large surfaces, like that which would occur between twin or triple windows, in the mullion, are securely braced inside with galvanized sheet iron or bar iron. =78.= In the construction of metallic sash, solder is never used for holding the parts together, for all parts must be either lock-seamed or riveted, the lock seams being illustrated at _g_, Fig. 36. Soldering may be used only to fill up the joints. The objection to a joint that is only soldered and not lock-seamed is that in a severe fire when the window is subjected to an intense heat, the joint is apt to open by the solder melting out. When the joint opens, flames may go through and the fire-stop will thus be soon destroyed.