How does one properly do this alkali melt? (Pfleger's indigo synthesis)
On academia, I found a reading of Industrial Dyes: Chemistry, Properties, Applications by Wiley (2003) that on first view sheds some light on the processes and chemistry.  Looks like an interesting secondary reference, rather than a Organic Synthesis.  I took some screen photos from there 



and



and



however, the concluding bibliography may turn out sour



as many of these references are likely only in German.

Perhaps one may trace either in these, or more recent work (than 1890) citing them provide a more detailed procedure.  

Addition: On archive, I found a "medium old" book about dye chemistry by Fierz, initially published in 1949, that comes pretty close to your intended alkaline melt; except omitting sodium amide:



Perhaps other old books dedicated to this topic equally tell more about these secrets, than modern textbooks?