We observed by optical microscopy the propagation of the thermal spin transition in single crystals of the archetype compound [Fe(btr)2(NCS)2]*H2O (btr = 4,4′-bis-1,2,4-triazole). In fresh crystals we could follow well-defined transformation fronts of various shapes. The propagation conditions revealed to depend very sensitively on the structural quality of the crystals. The best observations were obtained on cooling selected fresh crystals, rather thin, protected by an oil layer, under isothermal conditions. They evidenced the progression of a well-defined transformation front through the entire crystal. This is typical for a nucleation and growth process governed by inhomogeneous nucleation. The front shape was simple (circular or linear) and progression was regular in the best cases. Crystals of lesser quality (or already cycled through the transition) exhibited a more complex behaviour, assigned to the impact of internal stresses, either genuine or generated by the previous transformation of the crystals. These observations provide new insights into the physics of like-spin domains, which are briefly discussed.