The Collaborative International Dictionary of English v.0.48:

Slacken \Slack"en\, n. (Metal.)
   A spongy, semivitrifled substance which miners or smelters
   mix with the ores of metals to prevent their fusion. [Written
   also slakin.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slakin \Slak"in\, n. (Metal.)
   Slacken.
   [1913 Webster]