Indole is obtained by disolving pyrrole in 10 p.c. sulphuric acid, allowing the mixture to stand 1-2 hours, and then distilling in steam after adding excess of sodium hydroxide. Diethyl indole prepared by this process is a viscid ill-smelling oil, boiling at 270º-310º C (D. R. P. 125489).
Pyrrole may also be converted into indole by dissolving in dilute hydrochloric acid, adding slight excess of ammonia, filtering, extracting the filtrate twice with ether, and heating the tripyrroline obtained in the extract to above 300º C, when indole and pyrrole result (Ber. 1894, 476).
Dictionary of Applied Chemistry Vol 1, E. Thorpe 1921
http://www.ibiblio.org/patents/class/CLASS548.html (http://www.ibiblio.org/patents/class/CLASS548.html)
http://briefcase.yahoo.com/bc/beefiles2001/lst?&.dir=/Pyrrole+to+Indole&.src=bc&.view=l&.last=1 (http://briefcase.yahoo.com/bc/beefiles2001/lst?&.dir=/Pyrrole+to+Indole&.src=bc&.view=l&.last=1)
http://briefcase.yahoo.com/bc/beefiles2001/lst?.dir=/BerVol27p476-480&.order=&.view=l&.src=bc&.done=http%3a//briefcase.yahoo.com/bc/beefiles2001/lst%3f.dir=/BerVol27p476-480%26.src=bc%26.view=l (http://briefcase.yahoo.com/bc/beefiles2001/lst?.dir=/BerVol27p476-480&.order=&.view=l&.src=bc&.done=http%3a//briefcase.yahoo.com/bc/beefiles2001/lst%3f.dir=/BerVol27p476-480%26.src=bc%26.view=l)
http://briefcase.yahoo.com/bc/beefiles2001/lst?.dir=/Pyrrole+to+Indole&.order=&.view=l&.src=bc&.done=http%3a//briefcase.yahoo.com/bc/beefiles2001/lst%3f.dir=/Pyrrole%2bto%2bIndole%26.src=bc%26.view=l (http://briefcase.yahoo.com/bc/beefiles2001/lst?.dir=/Pyrrole+to+Indole&.order=&.view=l&.src=bc&.done=http%3a//briefcase.yahoo.com/bc/beefiles2001/lst%3f.dir=/Pyrrole%2bto%2bIndole%26.src=bc%26.view=l)
Bekanntlich geht der Verharzung des Pyrrols durch Salzsäure oder Schwefelsäure, wenn man nicht mit ganz concentrirter Säure arbeitet, zunächst vollständige Lösung voraus, die bei Verwendung verdünnter Säuren sogar ohne Gefahr durch vorsichtigen Erwärmen beschleunigt werden kann.
This looks like it says concentrated acid converts pyrrole into a resin, whereas dilute acid converts pyrrole to tripyrrole and careful heating can accelarate the process. Tripyrrole isn't specifically mentioned. I assume the resin isn't it, or is it? freetranslation.com gives this.
As you know the Verharzung of the Pyrrols precedes by salt acid or sulfur acid if one does not work concentrirter acid with entirely, first of all complete solution, that can be accelerated in use of thinned acids even without danger by careful heating.