I was heavily involved in transfers - first in a small way at Sowerbys around Coronation time then much more so in the 1960s with the various transfers used by Jobling on opal ware.
While you can usually get away with simply shoving the article into an annealing lehr, when it comes to anything like mass production there is rather more to it. A dedicated decorating lehr is used which is, in effect, an annealing lehr with a slow pre-heating section grafted on the front. Somewhere I posted a description of the DIY version which we made at Sowerbys at that time. A major difference with a proper decorating lehr is that the conveyor belt is returned outside the lehr so that it arrives back at the loading end cold. A normal annealing lehr has a "hot return" belt, brought back on the inside. From an engineering viewpoint these differences are not trivial. Belts are very heavy and heating (and cooling) them while travelling takes a great deal of energy.
As an aside, we had a case where, in the past someone had acquired a lehr which must have started life as a decorating lehr and modified it, hopefully, for annealing mass-produced pressed ware for later tempering. From time to time breakages occurred in the on-line testing of the final product and a few reached domestic kitchens (rice pudding all over the floor was the usual symptom). The cause was small, difficult to see cracks resulting from articles hot from the press being loaded on to the cold-return belt. A really powerful gas burner had to be fitted to pre-heat the belt to a temperature which it should have been at from the outset.
Back to transfers. Jobling made fluoride opal ovenware, decorated by transfers (as well as, in some cases, by direct silk screening) which were fired in a "proper" decorating lehr before being heated and quenched to temper them to give the equivalent resistance to thermal shock as borosilicate. The transfers had to be capable of withstanding all of this. Johnson, Matthey of course managed this very well, as did the limited number of transfers which we made ourselves. However, one small firm of transfer makers came badly unstuck with a complex transfer which used, I think, at least five different colours. Very precise printing registration was needed for success. Less precision resulted in the overlapping of several layers and the resultant extra thickness of enamel set up stresses which gave us article breakages.
Simple studio-type use of transfers is no doubt easy. When you start in big time there can be problems!!
Adam D.