Polyester, rayon and spandex: what the percentages tell you

Fabric composition is the most useful line on a listing and the one most people skip. On summer tops it tells you more than the product name, because the name describes a shape and the blend describes how that shape holds up on a hot day and after twenty washes.

The rayon mix: 60 polyester, 35 rayon, 5 spandex

This is the tank and tee blend and the majority of the catalog. Rayon is the reason these feel soft rather than plasticky, and it is what gives the fabric its drape, which on a flared cut is doing real work. Polyester supplies the structure and the crease resistance. The five percent spandex stops the shape going slack.

The trade is heat sensitivity. Rayon relaxes when it is hot and wet, then sets shorter as it cools. That is the mechanism behind a top losing an inch of length over a month, and it is entirely avoidable by keeping the blend out of a hot dryer.

The camisole mix: 94 polyester, 6 spandex

No rayon at all, and the difference shows up in the hand. This blend is crisper and lighter, and it holds a texture better, which is why the eyelet detail on the camisoles sits on this fabric rather than the rayon one. Eyelet needs the fabric around each opening to stay put, and rayon relaxes too much for that.

It also dries faster and creases less. What it gives up is the soft fall of the rayon mix, so a camisole in this blend reads a little more structured than a tank in the same shade.

The bandeau mix: 85 polyester, 15 spandex

Fifteen percent spandex is a lot, and on a strapless top it is structural rather than a comfort feature. There is no boning and no silicone strip on these. Ruching plus stretch is the entire mechanism holding a tube top where you put it.

Two things follow. The fit needs to be snug to work, so sizing up on a bandeau defeats the design rather than making it more comfortable. And spandex is the fibre that dies fastest at a permanent fold, which is why these should be hung or rolled rather than folded across the ruching.

Reading a blend you have not seen before

• Rayon above about 25 percent means soft and drapey, and means keep it away from heat.
• Polyester above about 90 percent means crisp, quick drying and resistant to creasing, with less give.
• Spandex at 2 to 6 percent is recovery. At 10 percent and up it is doing the holding.
• Cotton on a summer top sounds better than it wears. It holds sweat and stays damp, which is why this line does not use it.

None of this requires a textiles background. Three numbers on a label, read in that order, will tell you how a shirt behaves before you own it.