Cotton used in bedsheets generally falls into three varieties: Upland, the standard and the lowest-quality); Pima, a crossbreed between Upland and Egyptian, next-best to Egyptian; and Egyptian, which has the longest staple, the finest fibers and the best quality. Egyptian cotton is any cotton grown in Egypt and was traditionally the gold standard for cotton because of its long but fine staple, or fiber length. This creates both durability and extreme softness. Then short-staple cotton seeds were brought into Egypt and were easier to grow. Today the best way to get long-staple cotton sheets is to buy sheets labeled either long-staple Egyptian cotton or Supima cotton.
It's About the Weave
The two most popular choices for bedsheets are percale and sateen. These terms refer to the type of weave, not to the type of fabric. Both percale and sateen come from cotton, and both can come from any of the three varieties of cotton. Thread count refers to the number of threads in one square inch of fabric. The higher the count, the softer and stronger the fabric, and the finer the quality of the sheet.
Percale is a plain weave fabric, meaning that the warp threads, which run lengthwise, and the weft threads, which run horizontal, cross each other one at a time -- it is a simple one-over and one-under, 1:1 thread weave structure. This creates a matte, flat appearance. Because percale threads are tightly woven, these sheets have a fine, soft, powdery texture and finish.
Sateen sheets have an elegant and luxurious look and feel. They are made using a sateen weave: The warp threads, which run lengthwise, are interlaced with filling threads. The sateen weave structure is a multiple thread (usually three or four) over, and one under. The result is a lustrous, smooth-faced, slippery surface and a surprisingly durable fabric.
Egyptian Cotton and Mercerization for Sateen
Better-quality sateen sheets start with long-fiber Supima/Egyptian cotton. The cotton is then mercerized to give a high level of sheen. Mercerization involves soaking the fiber in a bath of sodium hydroxide and then in an acid bath. This makes the cotton fiber stronger and more conducive to dyeing. It also adds a luster to the fibers. Lower-quality sateen sheets are merely calendered, which produces a sheen that doesn't last long. This is not genuine sateen.
All 100-percent-natural fiber fabrics have a tendency to wrinkle, but natural-fiber sheets are much softer than any sheets that minimize wrinkling by the addition of harsher, synthetic fibers. Because percale and sateen both come from natural fiber materials, there is no discernible difference in wrinkling.
Sateen has a smoother, slick feel, "satiny" without satin. Percale has a softer, more powdery, "cottony" finish" Sateen holds in warmth, so it's a good winter bedding material. It's not particularly good for hot, humid climates without good air conditioning. Percale works best as a summer sheet.
Percale has a more durable weave than sateen because the simple one-to-one weave is less likely to snag or pill over the years. Lower-thread-count percales tend to be slightly more durable than higher thread counts, but the difference isn't large. Sateen is less durable, but Supima/Egyptian cotton sateen will last a good many years longer than cotton sheets from a lesser grade of cotton. The nature of the sateen weave, multiple threads over one, makes it more susceptible to pilling and snagging. Over time, sateen may develop a somewhat flannel-like appearance. Occasional ironing of sateen sheets will mitigate the tendency to pill and snag.