Winter is the time of year when a lot of activities take place indoors. Our regular sports technology feature looks at three of the most popular cue sports - snooker, billiards and pool - which all have one thing in common: the balls.
Before we go any further with this column readers might be encouraged to dismiss the advice of author Terry Pratchett: "A 'Thaum' is the basic unit of magical strength. It has been universally established as the amount of magic needed to create one small white pigeon or three normal-sized billiard balls."
As engineers, E&T readers will certainly know that billiard balls and those of other cue sports cannot be magicked out of thin air - they occur as the result of a highly technical manufacturing process.
And while billiards and the like may not appear to be the most physically demanding of activities, they are a little more dynamic than you might think.
For instance, a cue ball accelerates from zero to 30km/h in a fraction of a second when hit and can generate friction temperatures of up to 250degC between the ball and the table cloth, while a professional billiard player will walk up to 5km during a game as he/she circles the table and moves from table to chair.
There are some 50 types of recognised cue ball game, with snooker, billiards and pool being the most popular. The balls for each game vary. For instance, billiards is played with three balls of 61.5mm diameter and no standard weight, while snooker is played with 22 balls of 52.5mm diameter within a tolerance of +/-0.05 mm but no standard weight.
Balls for all cue sports were originally made from wood or clay, then ivory, since wooden balls were unreliable in terms of size, shape, weight and density for the obvious reasons that wood is not a material that possesses any of these features in a consistent manner, and clay was little better.
Ivory was a considerable improvement although still not perfect (especially from an elephant's point of view - by the beginning of the 20th century, up to 12,000 animals a year were killed to supply the British cue sports market alone). The elephant tusks, from which snooker and billiard balls were made, would vary in density along their length and this led to inconsistencies in weight and mass even among balls made from the same tusk.
In 1869 US inventor John Wesley Hyatt developed the cellulose nitrate billiard ball from the world's first industrial thermoplastic, Celluloid, which provided consistent shape, size and density but had the distinct disadvantage that in the production process it would occasionally explode owing to the fact that it is a volatile material.
This problem was eventually overcome by experimenting with other plastic compounds such as Bakelite and Crystalate. By the late 1920s plastic composite balls became common, and later polyester balls.
Today, Belgian company Saluc, which makes phenolic resin balls under the name Aramith, is the world's premier billiard, snooker and pool ball manufacturer, with 80 per cent of the global market. It is actually the only company to manufacture phenolic resin balls, a material which has the advantage of being far more resistant to impacts and scratching than polyester as well as lasting up to five times longer.
Saluc uses a phenolic resin specifically designed for cue sports balls, and production involves 13 stages over a three-week period, which include casting, curing, grinding and polishing.
Castings are made using injection moulding machines, which heat the liquid resin and pour it into flexible latex moulds, or forcibly inject the latex into the mould in order to prevent the formation of air bubbles and ensure that the ball is fully cast.
Some moulds are made to cast one ball at a time, with the mould peeled off after casting; alternatively a chain of balls may be cast, with the chain then broken apart and the balls given a smooth finish by machine lathes.
The whole process involves a combination of computerised technology and manual and visual checking to ensure the tightest tolerations and specifications, and each ball is checked manually before leaving the factory.
Aramith balls are manufactured in several varieties, with seven criteria controlling their production: balance, brilliance, colour, density, diameter tolerance, roundness and surface polish.
The higher the quality of the balls, the finer the grain of the phenolic resin used in their manufacture, which results in a better and longer lasting polish, while a series of hardening processes provide a transparent, vitrified surface.
Yves Bilquin, sales and marketing manager for North America, says that Saluc's carefully controlled production process "gives Aramith balls their specific reactivity and allows them to hold their high gloss polish over time, resulting in minimal ball and table-cloth wear".
While it would be natural to assume that ball diameter tolerance and roundness are the most important factors in the design and manufacture of a cue sports ball, it's actually the raw material used in their production along with molecular structure, density and rebound capacities which are the key elements.
The through-hardened phenolic resin used by Saluc results in a completely stable material with homogenous density dispersion throughout the ball and the centre of gravity exactly where it should be - at the centre of the ball - the result of which is excellent balance and a true and accurate roll.
As for rebound, which is an essential aspect of cue sports, the elasticity of the resin used by Saluc is calibrated to provide predictable rebound so that players can optimise their following shots.
And for balls that are numbered, as in pool, Saluc has them precision engraved in a solid core that runs all the way through the ball (polyester balls use a number 'plug' which can fall out over time). This engraving process makes it impossible for the number to fall out and ensures a longer lifetime for the ball.
Interestingly, should a Saluc pool ball eventually reach its breaking point (bearing in mind that this requires a five-tonne load) the homogenous structure of the ball means that it will break up at random rather than along the line between the colour stripes and the rest of the ball, as you might expect.
Other important characteristics of high-quality phenolic resin include less vulnerability to abrasive 'burn spots' from the surprisingly high friction temperatures between ball and table-cloth, and, of course, high impact resistance.
In short, the company's phenolic heat curing process results in balls with a vitrified, high-density surface which offers maximum impact resistance and shape retention. Independent tests performed by Vito Leo, PhD in materials science and solid state physics and associate professor at the Free University of Brussels, found that "when all kinds of polyester balls on the market are tested... both the cloth and the balls are greatly damaged after some time - some are even destroyed - whereas the Aramith balls... show an exceptional shape even after extensive longevity tests".
Another nice little touch that Saluc features on some of its pool balls, such as the Super Aramith Pro-Cup, is the addition of six red dots, which allows players to visualise the intentional and unintentional spin they apply to the ball.
Today Aramith balls are used by pretty much all professional players in pool, snooker, billiards and related disciplines. Organisations such as World Snooker and the United States Professional Pool Players' Association use them as the official balls in their tournaments.
Ivan Hirschowitz, press officer for World Snooker, told me that "there's no doubt Saluc are the best manufacturer of snooker balls and we are very happy with the relationship we have with them. So much so that there are no major changes planned in the technology used in ball manufacture at present, but we have been testing - with Saluc and others - a new polish for balls which is aimed at reducing 'kicks' and irregular cushion bounces, which has always been an issue in snooker".
Hirschowitz explained that in the tests the white cue ball was polished between frames and the rest of the balls once a day, and the cleaning polish was trialled recently in a competition in Lisbon and received very positive feedback from players.
There are now plans in hand to introduce the process to the professional game.