Complex sentence: Ovens were graded in warmth from around 70F (21C) to 100F (39C) and as drying proceeded trolleys loaded with trays were moved to progressively warmer ovens.
Split into simpler sentences: Ovens were graded in warmth from around 70F (21C) to 100F (39C). As drying proceeded trolleys loaded with trays were moved to progressively warmer ovens.