What is "scouring cinder"

Scour \Scour\ (skour), v. t. [imp. & p. p. Scoured; p. pr. & vb. n. Scouring.] [Akin to LG. sch["u]ren, D. schuren, schueren, G. scheuern, Dan. skure; Sw. skura; all possibly fr. LL. escurare, fr. L. ex + curare to take care. Cf. Cure.]

1. To rub hard with something rough, as sand or Bristol brick, especially for the purpose of cleaning; to clean by friction; to make clean or bright; to cleanse from grease, dirt, etc., as articles of dress.

2. To purge; as, to scour a horse.

3. To remove by rubbing or cleansing; to sweep along or off; to carry away or remove, as by a current of water; -- often with off or away.

   [I will] stain my favors in a bloody mask, Which, washed away, shall scour my shame with it.
   --Shak.

4. [Perhaps a different word; cf. OF. escorre, escourre, It. scorrere, both fr. L. excurrere to run forth. Cf. Excursion.] To pass swiftly over; to brush along; to traverse or search thoroughly; as, to scour the coast.

   Not so when swift Camilla scours the plain.
   --Pope.

5. To cleanse or clear, as by a current of water; to flush.

   If my neighbor ought to scour a ditch.
   --Blackstone.

   Scouring barrel, a tumbling barrel. See under Tumbling.

   Scouring cinder (Metal.), a basic slag, which attacks the lining of a shaft furnace.
   --Raymond.

   Scouring rush. (Bot.) See Dutch rush, under Dutch.

   Scouring stock (Woolen Manuf.), a kind of fulling mill.