Paper Terminology When choosing paper for the classroom it can be hard to know what all the various terms mean. Here is a list of some of the more common questions you might have about paper. What is GSM & UM (µm)? GSM and UM (µm) are units for measuring the weight and thickness of paper. GSM is based on weight and stands for Grams per Square Metre. In most cases a higher GSM means a thicker paper, for example standard cartridge paper is 110gsm and standard watercolour paper is 300gsm which is substantially thicker. UM is based on thickness and stands for micrometres. There are 1000 micrometres to 1 millimetre. To convert UM to mm simply divide UM by 1000. What is Paper Tooth, Hot Press & Cold Press? These all refer to a paper’s surface texture. The tooth of a paper is how rough the surface is. If a paper has a lot of tooth it means the paper has a very rough surface. Tooth on paper helps some media to cling on to the surface e.g. dry media such as pastel. Hot press and cold press are process’s resulting in a surface texture, usually in the manufacture of watercolour paper. Hot press is a smooth surface and cold press is a medium rough surface. What is Acid Free & PH Neutral? The terms acid free and pH neutral are interchangeable and usually refer to archival or artist quality papers. What this means is the acid in these papers has been balanced and will not yellow or degrade. This is important when mounting photographs or when preserving artworks. What is Stretching Paper? Stretching is a way to prepare watercolour paper before use. This stops the paper from buckling when using water whilst painting. To prepare paper, soak it in a water bath then tape to a board and allow to dry flat. What is Paper Grain? Like wood, paper has a grain; this is important when tearing and folding is required. Tearing along the grain produces a neat and straight edge. Tearing against the grain causes an uneven and messy edge. Folding along the grain will also produce a clean, smooth and flat fold. Folding against the grain will cause fibres to crack, creating an uneven fold. What is Rag? Rag paper is made of cotton or a combination of cotton and wood. If a paper is referred to as rag it’s usually 100% cotton. Rag papers are used mostly for watercolour but are also commonly used in collage works. What is Easel Paper? Easel paper is a generic term for bulky news print that is larger than 38 x 51cm and fits on an easel. Easel paper is economical and can be used for painting, drawing as well as life drawing sketches. What is a Ream? A ream is the term for 500 sheets of paper.