Most trade paper books have coated covers; that is the paper has an enamel coating to print a smaller dot than uncoated paper. Since most covers are printed in four color process (CMYK) which relies on dots from 150 lines per inch (lpi) to 300 lpi, accurate color reproduction depends on a papers ability to control the size of the dot.
Coated papers are available as coated on one side (c1s) or coated on both sides (c2s). Since most trade paper covers have no printing on the inside covers, c1s, as the less expensive of the two, is most commonly specified.
Early on, the paper was described not by its basis weight but by its caliper (thickness). Since the paper that is enamel coated can be of varying weights, an 80# c1s might caliper at .010” or .012”. Designers and printers needed a better descriptive process for cover paper selection and adopted the paper caliper as a reasonable identifier. So ,while a .010 pt. c1s always calipers as .010 inch, its basis weight can range between 122 pound to 133 pound depending on the original paper weight and how much coating was applied.
Saddle stitched books generally use a .080" or .010" coated stock on one or both sides. A thinner paper works well since the cover is fully folded at the spine.
Glue bound books tend to use .010 or .012 coated stock for a more substantial cover. And while thicker coated sheets are available (generally called “blanks”), sheets like a .015 simply don’t respond to the necessary bending and scoring, although they’re fine for mechanically bound books.
Traditionally printers discouraged the use of c2s papers when the book was glue bound, but most modern glues adhere satisfactorily to enamel coating. If you’re printing the inside covers and want to use a c2s paper, check with the printer first.
Uncoated paper generally can’t reproduce full color as well coated paper but offers a wide variety of colors and finishes for a creative designer. While the most popular uncoated covers are 65# and 80#, many designers have discovered a paper type known as bristol (actually printing bristol), which is visually similar but usually produced with shorter fibers. Bristols have their own basis size and weight, making a 65# cover paper the equivalent of an 80# bristol. Bristols are the paper stock of choice for enamel coating to create c1s and c2s cover paper.
While protective coatings, metallic and fluorescent inks are not necessary or practical on uncoated paper, skilled designers take advantage of the rich colors and textured finishes to emboss, foil stamp and even die cut stunning visual effects for covers.
As with thicker coated stocks (blanks) being of limited use for trade paper, uncoated stocks are available in extremely heavy weights, usually as duplex papers, two sheets bonded together to double the paper weight (two 65# cover sheets will make one 130# sheet). Some duplex even have contrasting colors back to back for creative effect.
Paper grain exists because paper is made by moving water and pulp in one direction through all of the manufacturing processes, thereby aligning the pulp fiber in one direction called the grain.
Since glue bound covers are scored before they are folded around the book block, grain direction is of less importance than saddle stitch covers, which are folded sharply in half. Heavy cover stocks may tend to crack along the fold and all cross grain cover stocks will crack and sprout short fiber “whiskers” along the fold. That’s why magazines that are saddle stitched tend to have light weight cover stocks or even text stocks for their covers.