Pressing and ironing are different operations and one of them stretches cloth

Two motions, two results

Ironing is sliding. The plate moves across the cloth under pressure, flattening it as it goes. Pressing is lifting and lowering: the iron is set down, held, and picked straight up again before being moved somewhere else.

The difference sounds pedantic and it is not. A hot iron dragged across damp fabric applies a shearing force, and damp fibres under heat are unusually willing to move. The cloth stretches in the direction of travel, and it does so a little at a time, invisibly, over many sessions.

Anyone who has watched a tailor work with an iron will have noticed it never slides on wool. It goes down, it sits, it comes up. The garment is moved instead.

What heat and moisture are actually doing

Fibres hold their shape because of bonds within and between the molecular chains inside them. Water and heat temporarily loosen those bonds, which lets the fibre take a new configuration; when it cools and dries, the bonds re-form and the new shape sets.

That is the whole mechanism, and it explains almost everything about pressing. It is why steam does more than heat alone. It is why the crease you set will not hold until the cloth has cooled — lifting a garment while it is still hot and damp and hanging it up is how a freshly pressed crease vanishes.

It also explains why the process is reversible on some fibres and not on others. Wool responds beautifully and can be reshaped repeatedly. Cotton and linen take a crease well and give it up readily. Synthetics are thermoplastic, so heat softens them into a new shape permanently, which is what a permanent pleat is and why a scorch cannot be undone.

Shine, and why a pressing cloth exists

Press a wool garment directly with a hot iron and the surface can go shiny. The mechanism is simple flattening: the raised fibre ends that scatter light get pushed down into a smooth plane, and a smooth plane reflects light instead of diffusing it.

On synthetics it is worse, because the fibre surface actually melts slightly and then sets glossy. In both cases the shine appears first exactly where there are layers underneath — seam allowances, pocket bags, hems — because those spots take more pressure.

A pressing cloth is a piece of plain cotton or linen laid between the iron and the garment. It spreads the pressure, keeps the sole plate off the fibres, and lets steam through. It is the single cheapest way to stop damaging clothes, and almost nobody owns one.

Press the wrong side, and press on the right shape

Most pressing should be done from the reverse of the garment, which puts all of the risk on a surface no one sees. When the right side has to be pressed, that is what the cloth is for.

Shape matters as much as heat. A garment is three-dimensional and an ironing board is flat, so pressing a curved area on a flat surface flattens the curve out of it. This is why tailors use a ham — a firm, curved, tightly stuffed pad — for darts, shoulders, sleeve heads and the roll of a lapel.

The lapel is the clearest case. A jacket lapel is supposed to roll softly from the collar to the buttoning point. Pressed flat with a crease down the fold, it looks folded rather than rolled, and putting the roll back takes steam, patience and a curved surface.

Steaming does a different job than pressing

A steamer relaxes fibres with moisture and lets gravity pull the wrinkles out. There is no pressure involved, so nothing is flattened, nothing is stretched, and no shine can occur. For wool, knitwear and anything with a raised or textured surface, it is usually the right tool.

What steaming cannot do is create a sharp edge. A trouser crease, a shirt collar, a knife-pleat — those need heat, moisture and pressure together, and then cooling under that pressure. Steaming a shirt gets it presentable, not crisp.

Between the two of them most garments are covered. Structured tailoring and shirting want a press; knitwear, jersey and anything textured wants steam; and a great many things want neither, because hanging them in a bathroom while a hot shower runs will do.

Small habits that save garments

Test the temperature on a hidden seam allowance first, especially on anything blended, since the lower-melting component sets the limit and the label does not always mention it. Work upward from a low setting rather than downward from a scorch.

Let things cool flat before you move them, and hang them only once they are dry to the touch. This is the step most often skipped and it is where half the work is lost.

And press less overall. Pressing is heat and pressure applied to cloth, and every session is a small withdrawal from the garment's life, exactly as washing is. Clothes that are hung properly when taken off need far less of it.