Slag \Slag\, n. (Metal.) A product of smelting, containing, mostly as silicates, the substances not sought to be produced as matte or metal, and having a lower specific gravity than the latter; -- called also, esp. in iron smelting, cinder. The slag of iron blast furnaces is essentially silicate of calcium, magnesium, and aluminium; that of lead and copper smelting furnaces contains iron.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Cinder \Cin"der\ (s[i^]n"d[~e]r), n. [AS. sinder slag, dross; akin to Icel. sindr dross, Sw. sinder, G. sinter, D. sintel; perh. influenced by F. cendre ashes, fr. L. cinis. Cf. Sinter.]

1. Partly burned or vitrified coal, or other combustible, in which fire is extinct.

2. A hot coal without flame; an ember. --Swift.

3. A scale thrown off in forging metal.

4. The slag of a furnace, or scoriaceous lava from a volcano.

Cinder frame, a framework of wire in front of the tubes of a locomotive, to arrest the escape of cinders.

Cinder notch (Metal.), the opening in a blast furnace, through which melted cinder flows out.

Source: Webster's Revised Unabridged Dictionary (1913)