The performance of cross country skis is directly and significantly influenced by the glide wax applied to the bases. Properly waxed skis will have an enhanced glide that not only allows for faster skiing with less effort, but also yields skis that turn and descend with increased control.
There are many different types of snow: wet, moist, dry, new, old, powder, crusty, slush, windblown, etc. Each type of snow reacts differently with the ski base that glides across it. Skis are made with a base material that can be fine tuned to match the various snow conditions by simply changing the wax. The bases of modern skis are porous, like micro sponges. Different types of waxes can be melted into the bases so that the skis will have optimum glide in each snow condition.
Ski bases are made of polyethylene - similar to the plastic used in many food containers. But rather than being extruded to form an impervious sheet, finer particles of polyethylene are polymerized into a crystalline material by using heat and pressure in a process called "sintering". In between the chemically and physically bonded crystals are gaps and voids - empty spaces where the wax can be absorbed. Initially, these gaps contain amorphous (noncrystalline) polyethylene, as well as carbon particles in black "graphite" bases.
Ski waxes are blends of paraffins (candle-like waxes) and micro-crystalline and noncrystalline polyethylene (plastic waxes). Paraffin and polyethylene waxes melt at about the same temperature and blend easily. This is why a soft paraffin wax used for a ski's first waxing(s) will result in good absorption - the paraffin wax blends with the residual amorphous polyethylene left within the gaps as a result of the sintering process.
When a ski moves across snow, the snow crystals are melted at the point of contact and the ski glides on a thin film of water at each contact point. The ideal temperature for this to occur is approximately 26º F. As the temperature drops, less snow is melted and dry-slide (solid-to-solid friction) principles become increasingly important. Paraffins do not dry-slide well. Sharp pointed snow crystals tend to stick into the wax and slow the skis. Plastic (polyethylene) waxes are much harder and become the predominant component of waxes designed for cold dry snow, especially when newly fallen.
As the temperature rises above 26º F, too much water tends to be melted under the ski and capillary attraction between the ski base and the water increases. Each globule of water tries to "hold on to" both the base and ice crystal(s) from which it melted. It takes energy to break the tension caused when the globule is stretched between the moving base and the stationary ice crystal. Softer paraffin waxes help bases repel the water and are the choice for wet and warmer snows.
There are several additives found in today's ski waxes, the most notable of which are graphite and fluorocarbons. Quality black graphite bases contain about 8% carbon, found within the gaps of the base's crystal structure. Graphite reduces the build-up of static electricity and the resultant electrical attraction between the base and snow. Graphite also transfers heat, caused by friction at the base surface, towards the center of the ski. In warmer conditions, this helps prevent excess melting of the snow and the build-up of thicker water films (globules), minimizing capillary attraction and water tension. Part of the reason for the clear (non-graphite) base at the tip section of the ski is to allow for initial production of water films at the snow/base contact points. The graphite in the remainder of the base helps prevent excess water buildup. The graphite particles in ski wax work in a similar manner by increasing the carbon content within the gaps of the surface layer of the base.
Fluorocarbons are carbon molecules that have had their hydrogen atoms (on the hydrocarbons) replaced with fluorine atoms (thus becoming fluorocarbons). Fluorine is the most electronegative of all elements. It is very hydrophobic (water hating). Fluorocarbons give increased glide in moist and wet snow conditions by reducing capillary attraction between the water and the base. In dirty snow conditions (e.g., spring skiing), dirt particles (which tend to be negatively charged) are also repelled, keeping the bases cleaner. Dirt on the bases significantly increases friction and slows the skis.
There are three basic types of fluorocarbons used in ski waxes: short molecules that are polymerized and larger wax-like molecules. Teflon, or polytetrafluoroethylene (PTFE), is made by polymerizing (repeated linking) molecules with two carbon atoms surrounded by fluorine atoms. Many waxes like Maxiglide (liquid, paste, and speed powder), Swix F4, Ski-Go 280, and Star C2, contain PTFE. This is an excellent low friction compound which can reduce surface tension by upwards of 40-50%, especially in moist to wet snow.
The second type of fluoro compounds are long chain carbon molecules. The "per" of the term "perfluorocarbons" means that all the hydrogen atoms have been replaced with fluorine. The exact properties of the fluorocarbons can be altered by varying the carbon molecule structure (carbon to carbon atom bonds) and the purity or blend of the final compound. This allows the chemist to develop additives for different snow conditions and temperatures, and explains why (or how) there are different types of "F" powders and block waxes. 100% Perfluorocarbon powders and solids are expensive, but offer exceptional glide in moist to wet snows.
The third type of fluorocarbons are hybrids with half of the molecule being a hydrocarbon and the other half being a fluorocarbon. Hydrocarbons, which are electrically neutral, do not like to mix with fluorocarbons, which exhibit a negative charge on their surface. Mixing pure molecules of these compounds would be similar to trying to mix oil and water... the molecules would separate. By making a hybrid molecule, the hydrocarbon end will mix with the hydrocarbon paraffins and synthetic (polyethelene or plastic wax) components. It is possible to mix into the bar or block fluoro waxes upwards of about 16% fluorocarbons. At least 3% fluorocarbon is needed to obtain the water repelling benefits of the fluorocarbons. Low fluor bar waxes contain 1-6% fluorocarbons (not all "low fluor" waxes are equal), mid range about 6-10%, and high fluor bar waxes contain 10-16%. Keep in mind that the 86% non-fluoro components add significantly to the glide speed of the wax and that fluorocarbons are important only in moist and wet snow conditions. Using high fluoro content waxes in dry snow is simply a waste of money.
Fluorocarbons should not be confused with ChloroFluoroCarbons, or CFC's, the chemicals that cause damage to the ozone layer in the upper atmosphere. (It's the chlorine atom of CFC which is the culprit.) The hazard of the smoke produced at high iron temperature applications of powdered perfluorocarbons is not an issue with the block fluorocarbons. This is because block fluorocarbon waxes are melted in at a lower temperature. Just like all regular waxes, fluoro block waxes should be applied at an iron temperature that does not cause the wax to smoke. If the wax smokes, the iron is too hot and should be turned down. Cheap irons, especially lightweight travel models, usually do not work well for any of the harder (plastic) waxes, cycling between being too cool to melt the wax, to being so hot the wax ends up smoking. The solution is to buy an iron designed for waxing skis, not clothes, such as the professional irons from Star or Swix.
Snow contains varying degrees of "free moisture" or liquid water on the surface of the ice crystals. Moisture in the snow is dependent on temperature and humidity. Colder snow tends to be drier, with most of the water molecules locked into crystalline structures. As temperature rises, more and more water molecules tend to dissociate from the crystals, to be freed-up into liquid form.
Relative humidity (RH) is a measure (as a percent) of the water vapor in the air relative to the total amount of water the air can hold at a given temperature. Moist air (above 60% RH) tends to increase the free moisture content of the snow. The snow melts easier under the ski, in essence acting like the temperature is warmer. On the other hand, dry air (below 40% RH) tends to remove free moisture from the snow (causing it to evaporate). Skis perform like the snow is colder than the air and snow temperatures indicate. This is especially true on a windy day. The snow is literally blown dry.
Colder air can hold less moisture than warmer air. Relative humidity rises as temperature drops (the amount of water vapor in the air remains the same, but the humidity, relative to the colder air's lower water holding capacity, goes up). Clear skies at night allow surfaces to radiate heat to outer space. If these surfaces (car windshields, snow and ski tracks, etc) become cold enough, the relative humidity at these surfaces exceeds 100%, water from the air condenses on them, forming dew. If the dew point temperature is below 32º F, frost forms.
Of interest to skiers is that as the air temperature rises the next morning, the air becomes increasingly dry. It no longer has the moisture in it that precipitated as frost from the previous night. This dry air keeps the snow dry, pulling free moisture from it. This means that the snow not only gets really cold during clear nights, with new sharp-pointed crystals, but the snow also acts as if it is a few degrees colder because of the lower air humidity during the morning hours.
Choosing the proper wax may not be as simple as reading the temperature of the outside (air) thermometer. Snow temperature and relative humidity are just as important. Therefore, an understanding, or at least a consideration of, weather and atmospheric conditions is also important. Consider the following examples:
COLD CLEAR NIGHTS - wax for colder drier snow, especially if skiing before noon. It usually gets cold and dry after a storm (if the skies are clear). When in doubt, wax on the cold side. Snow temperature can lag 10-20ºF behind that of the air.
EXTENDED OVERCAST SKIES - when day and night temperatures are similar, snow and air temperatures will be, too. Humidity will be on the higher side, very high near unfrozen lakes and seas. Wax as if it's a bit warmer.
MILD NIGHTS, NO FROST - wax for warmer moist snow. Expect wetter snow on sunny warm afternoons, and higher humidity and moist snow on overcast days. It usually warms up just before a storm (air is compressed and warmed by the incoming front). Wax close to air temperature, hedging to the warmer side.
DURING A STORM - humidity is high. Wax on the warm side. Air temperature and snow temperature will be about the same. If cold, use plastic waxes.
BOWLS AND LAKE BEDS - cold air drops; wax colder for morning skis, but sunny afternoons might require a warmer wax for wet snow. If foggy, wax warmer.
SHADY AREAS, NORTH SLOPES - lack of sun means snow stays drier in the afternoon and the crystals stay sharper longer. Wax colder.
WINDY CONDITIONS - snow will be drier as moisture is removed by the wind; wax colder. Static electricity probable; use graphite waxes.
TEMPERATURES IN TEENS & BELOW - use harder plastic waxes. If the humidity is high (especially near large open bodies of water), use fluoro waxes.
TEMPERATURES IN THE 20's - lots of waxes work. Pay attention to humidity and night temperatures. Fluoros help; wax on the cold side since they can handle the extra moisture as (or if) the temperature warms.
TEMPERATURES ABOVE 30º F - fluorocarbons rule, especially in old snow.
NEW SNOW - fresh fallen dry snow (within the last 24 hours, or even several days if freshly tilled) will have sharp crystals. Plastic waxes work best.
OLDER SNOW - crystals are duller but can vary from abrasive if dry or cold (use plastic waxes) to being moist to slushy (use fluoro waxes).
NEW WET FALLEN SNOW - yuk! Use softer paraffin waxes (usually yellows), especially the ones containing fluoro. Put lots of structure on your skis. (Structure? See tech tip #1 in the archives.)
SIERRA NORDIC carries a complete line of glide waxes from Rode and Star, as well as selected waxes from Toko. Stop by for a demonstration in glide waxing technique at one of our free wax clinics.
All recommended waxes, tools, and supplies may be ordered from our On-Line Shop.