Why Coat a Ski in Something Slippery and Also Sticky? Two Opposite Jobs

A ski does not slide on ice directly but on an extremely thin film of liquid water between the base and the snow, generated by pressure and by friction as the ski moves. Managing that film is what wax does. Too little water and the base drags against dry crystals, while too much and the ski is held by suction, the way two wet sheets of glass stick together. Wax alters how water behaves at the surface, repelling it so that it forms a film of the right thickness rather than wetting the base completely, and it also fills and seals the porous structure of the base material, which absorbs wax and holds it.

Selecting a wax is a matter of matching several conditions at once:

• •Air and snow temperature, which is the primary variable
• •Crystal shape, since fresh sharp crystals behave nothing like old rounded ones
• •Moisture content, from dry cold powder to saturated spring snow
• •Humidity, which determines how much water is available at the surface
• •The dirt in the snow, which is considerable on a busy piste
• •How long the run is, since wax wears off progressively

Cross country skiing in the classic style requires the opposite behaviour in the middle of the ski, which must hold still against the snow while the skier pushes off and then release cleanly as the ski glides forward. That is achieved by a soft sticky wax applied only under the foot, whose consistency is chosen so that snow crystals penetrate it momentarily when the ski is weighted and release when it is not, which depends on matching the softness to the sharpness of the crystals. Snow near the freezing point defeats this, since the crystals are rounded and wet, so a thick paste is used instead. Skis with a patterned base that grips mechanically avoid the whole problem and glide slightly less well.

Wax is only half the system, because the material it is applied to does most of the work. Bases are made of a high molecular weight polyethylene that is deliberately porous, so it absorbs wax into the structure rather than merely carrying a film, and a base that has never been waxed slides poorly however good the snow. The surface is also textured with fine grooves running lengthwise, which break the suction that a perfectly smooth base would create on wet snow, and the pattern is chosen for conditions, with deeper texture for wet snow and finer for cold dry. Racing teams maintain several pairs of skis textured differently and select on the morning. Grinding to restore that texture is a specialist operation done periodically.

Racing waxes for decades contained fluorinated compounds, which repel water far more effectively than hydrocarbon waxes and gave a substantial advantage in wet conditions. Those compounds belong to a family that does not break down in the environment, accumulates in living tissue and has been associated with a range of health effects, and testing found elevated levels in the blood of wax technicians who had applied them in poorly ventilated conditions for years, along with contamination of snow and soil around race venues. The international skiing federation banned them from competition from the 2023 season, with enforcement by testing at events, and manufacturers have developed replacements that perform respectably and not identically.

A ski slides on a thin water film, and wax controls how that film behaves, preventing both dry dragging and suction. Temperature, crystal shape, moisture and dirt all determine which to use. Classic cross country skiing needs the opposite behaviour under the foot, using a soft wax that snow crystals penetrate when weighted and release when not. Fluorinated racing waxes were banned from competition from 2023.