On Food and Cooking

Then either stir the chocolate as it cools
further, until it thickens noticeably (an
indication of crystal formation), or pour
a portion onto a cool surface and scrape
and mix it until it thickens, and return it
to the bowl. Then carefully raise the
temperature of the chocolate to the
tempering range, 88–90ºF/31–32ºC, and
stir to melt any unstable crystals that
might have formed during the stirring or
scraping.
To seed melted chocolate with stable
crystals, chop and hold in reserve a
portion of solid tempered chocolate. Heat
the chocolate to be tempered to
120ºF/50ºC to melt all crystals, and cool
it to 95–100ºF/35–38ºC, just above the
temperature range in which stable
crystals form. Then stir in the solid
portion with its stable crystals, while
keeping the temperature in the tempering
range, 88–90ºF/31–32ºC.