Some cloths need a "cool" iron. Heat, as well as cloth, is chemistry. Synthetics responds very different than natural cloth under different temps. The amount of pressure from the weight of the iron matters, too. Thin cloth doesn't need much weight, and skippy Synthetics, practically nothing. A 20 pound iron and shirt cotton you might be holding up 10 to 15 pounds off the cloth. To flatten some seams you need more weight, so you let some more weight settle down. The old tailors had a lot of muscles. When you look at home sewers irons you see a dial to adjust the temperature for different cloths and the entire weight of these irons are skimpy. The old tailors learned how much weight and heat and how to use press cloths and when to use water and how much water to use. Many tailors today use 6 pound irons because the cloths are so light. If they were making dresses the iron might be even lighter. Some tailors are artist with the iron.