The springs at Petriolo were known to both Etruscans and Romans, the curative powers of the thermal baths have been celebrated ever since antiquity by the likes of the Roman statesman Cicero.
Petriolo was praised by the writer and poet Folgore da San Gimignano (1270 circa – 1332 circa) in his Sonetti de' Mesi:

E di novembre Petriuolo, il bagno, / con trenta muli carchi di moneta: / la ruga sia tutta coverta a seta; / coppe d’argento, bottacci di stagno: / e dar a tutt’i stazzonier guadagno; / torchi, doppier che vegnan di Chiareta; / confetti con cedrata de Gaeta: / e béa ciascun e conforti ’l compagno. / E lo freddo sia grande e ’l foco spesso; / fagiani, starne, colombi mortiti, / lèvori, cavrioli rosto e lesso: / e sempre aver acconci gli appetiti; / la notte ’l vento e piover a ciel messo: / e siate ne le letta ben forniti.

Near the bridge on Farma, one can still admire the ancient spa building, which consists of an indoor space which is only open on one side with a portico with four arches, called "porch of Pius II", named after Pope Enea Silvio Piccolomini, who stayed in Petriolo in 1462.
If you are looking for the best free thermal baths in Tuscany, I would prefer the to spend some time in Saturnia or Bagno San Filippo. Both sites, with the Cascate del Gorello and the Fosso Bianco in Bagno San Filippo are magical.