Herewith some curves of temperature against resilience measured during November 1989. The balls tested were as stated on the graph, Jaques Eclipse Premium and Barlow XT.

In passing, the balls tested by Jack Osborne in the U.S.A., which behaved like ping-pong balls at elevated temperatures probably had at least 70 - 90% rubber compound such as Butyl, Nitrile, Silicone or other mixed with some cross linking agents to "set" the matrix and harden it to the requisite bounce value at say 25°C. Further than this I cannot guess, because there are many other possibilities.

Certainly the U.S.A. balls behaved differently to Jaques or Barlow which both show a negative temperature coefficient of resilience under hot conditions.

1 - Jaques Eclipse Premium, 2, 3, 4 - Barlow
Current limits are 30-45" rebound height

Of interest is the rather rapid increase in bounce height of the Jaques ball below 10°C. This may explain why Jaques balls are normally between 1 and 3 inches below specification when measured here [South Africa] whereas in the U.K. without special precautions they could have passed.

The Barlow XT, specially developed to mix the physical properties while retaining proper chemistry of bonding, shows an interesting point of inflexion around 24°C. Both balls would presumably still pass specification (Jaques needs to be brought up 0.5") at 50°C, and certainly Barlow XT would pass at 0°C if the ball average bounce is brought down by 0.5".

In fact over the range 5 - 45°C, the tested range, with 6 points of measurement on the way, the Jaques balls change by 6.5", while the Barlow XT changes by 4.2".

To control or prevent the marketing of balls which possess high temperature coefficients of resilience the Association will have to set a temperature range on the specified limits of bounce. Looking at the supplied curves, it seems reasonable that the temperature range could be reduced from 5 - 45°C to 10 - 45°C. Taking into account variations between balls, both Jaques and Barlow XT should pass this specification, and it probably corresponds to the reasonable extremes in climate experienced normally by players around the world.

I am quite certain that both Jaques and Barlow as well as the many others can still further tighten their own quality control and balance of physical properties.

Lastly, two points of interest.

1. Testing must continue on agreed apparatus under independent laboratory control before new balls can be accepted. Even Jaques and Barlow plus others who may be achieving constant resilience may come up with further improvements which should be assessed by the Association itself or accredited test laboratories.
2. The tests were reassuringly accurate and reliable. Using ,a single spot bounce point on each ball, the repeatability of the results was so good that no series of 4 - 5 bounces at any set temperature varied by more than about 0.2" which is remarkable. This applied to all the balls tested.

The compounded and averaged results of 4 balls were used for the 100% plastic curve, with variation of 1.50 between maximum and minimum single ball resilience.

The Barlow XT test was done on 2 balls, black and red, simultaneously with the Jaques Eclipse Premium. The two Barlow balls followed exactly, repeat exactly, the curve shown, but were spaced exactly 0.51" either side. In other words, the bounce of the red at 15°C was 35.3", while that of the black was 34.3" and so on.