Period language in "12 Years a Slave": "soft soap", "clean shirt"

[Joel S. Berson]

To comment on only the language used in "12 Years a Slave", there was
just one scene where my ears pricked up:

"soft soap" ("to flatter"; the OED2 definition needs some
expansion).  OED2 has in 1840, and 1843 from Bartlett's _Dict. of
Americanisms_.

"[come in with a] clean shirt" (morally or ethically spotless).  Not
in OED yet!?  (Although my meagre printed resources and a very
superficial Google search also don't turn this up -- have I created
something out of whole cloth?)

Solomon Northup was kidnapped in 1841, and published his memoir in
1853.   So "soft soap" is possible, and in fact it appears in an 1855
edition of "Twelve Years a Slave" (GBooks, full view).

"Clean shirt" doesn't appear (in that edition).  When did it arise in
the sense I am supposing?

Joel

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