What is the minimum speed a glider can thermal at?
I do a lot of soaring and you thermal at min sink +/-, which is normally just few knots above stall, around 1.1 Vs.  The slower you can go the better (smaller circle) and sometimes going below min sink is worthwhile if it lets you core the thermal better, but your ability to do this can be limited by the glider's particular stall characteristics and your own flying skills.  

On gliders with docile stall behaviour you can pretty much circle on the edge of the stall buffet.  A glider that breaks sharply with not a lot of warning, and wants to roll off, you need to leave more margin lest you enter a spin while above someone else and you might thermal at min sink or a little above.

The Schweizer 1-26 has very docile stall behaviour with a very strong pre-stall buffet that starts well before the stall break, and you can thermal a 1-26 right in the buffet range, shuddering and shaking as you go around. 

BTW the best thermals come from lighter brown fields, that are dry.  Moisture in the ground makes it store heat and this tends to move the thermal activity to later in the day, and makes it milder.  Spring time after a very dry winter with little residual snow cover to moisten the earth when it melts will yield boomer conditions on good soaring days.