Selecting competition boules – hardness

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With respect to hardness (dureté), boules are generally described as —

• hard (dure, “D”)
• semi-soft (demi-tendre,”½T”)
• soft (tendre, “T”)
• very soft (très tendre, “TT”, or even super tendre)

The problem with these labels is that they conceal the fact that there are two quite different kinds of “hardness”. So …

What is hardness?

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Elasticity

Elasticity is basically a way of describing how “bouncy” a boule is. When a boule hits the ground or another boule, the pressure of the collision deforms the boule. A boule’s elasticity is its ability to recover from such a collision and return to its original shape.

A boule’s “elastic limit” or “yield point” is the greatest amount of pressure that it can take without breaking or being permanently deformed. Engineers determine a boule’s elasticity by putting the boule in a big press and mashing it under greater and greater pressure until the boule breaks or stays permanently mashed out of shape. The pressure at which that happens is the boule’s elastic limit.

In the USA we usually express pressure (tire pressure, for instance) in pounds per square inch (p/i² or PSI.) In France, which uses the metric system, boules manufacturers express the pressure of a boule’s elastic limit (or elasticity) in kilograms per square millimeter (kg/mm²). The higher this number is, the more bouncy a boule is. A very “soft” (low-bounce) boule has an elasticity of 110 kg/mm², while a very “hard” (high-bounce) boule has an elasticity of 140 kg/mm².

When Obut talks about its boules’ “hardness”, it is talking about its boules’ elasticity (aka rebound, aka bounciness).

Scratch resistance

The technical term for “scratch resistance” is “penetration resistance” or “indentation resistance”. For our purposes, this is a measure of how likely it is that your boule will get scratched or gouged if it hits a sharp rock on the terrain.

Engineers test the scratch resistance of a boule by taking a diamond with a standard angle of sharpness and pressing it into the boule with a standard amount of pressure. This makes a hole in the boule. The depth of the hole is a measure of how hard the boule is— the softer the boule, the deeper the hole. The standard way to express penetration resistance is a dimensionless number called the Rockwell C hardness index (HRC, or HrC). For safety reasons, the FIPJP requires all competition boules to have an HRC between 35 and 55. (A nice Youtube video explaining HRC is HERE.)

Is there any correlation between elasticity and scratch resistance?

Yes. There is a rough correlation. Or at least there used to be, before manufacturers started playing with new steel alloys and “anti-rebound” technologies (see next section). Back then you could map an elasticity number to a scratch resistance number. Here are the numbers, with softer at the top and harder at the bottom.

Boules that are both hard and soft

Generally speaking, petanque players want boules that are both hard (scratch resistant) and soft (low-bounce). To satisfy this desire, skilled boule-engineers have devised ways to make boules that are both hard AND soft.

The exact details of how they do this are trade secrets. Different manufacturers do it different ways. One way is to use special steel alloys and special tempering techniques. When a manufacturer does this, they may describe the boule as being made of “SPECIAL carbon steel” or “carbon steel ALLOY” (allié au carbone).

A manufacturer might also redesign the boule’s surface and/or interior structure in ways intended to reduce bounciness. This is how MS Petanque does it with their MS 2110 boules.

“These boules are made with internal ribs that reduce the shock of impact, giving low bounce and reduced rebound. Their hardness makes them resistant to wear.”

These are called “anti-rebound” or “controlled rebound” technologies.  Basically the goal is to create boules that have (as Obut advertises) “the playing behavior [comportement au jeu] of a ‘very very’ soft boule, while boasting a wear resistance comparable to that of a traditional half-soft boule.”

• Obut invented a new term for such boules— dureté+, or in English “Hardness +”. (You can see the label dureté+ in the first two pictures in this post, which come from the Obut web site.) Obut’s high-end dureté+ boules are the RCC and RCX, where “RC” stands for Rebond Contrôlé— controlled rebound. The final “C” indicates carbon steel, while “X” indicates inox, i.e. stainless steel.
   
• MS Petanque offers the “MS 2110 anti-rebound” model” (pictured above). (The “110” part of the name indicates the boule’s elasticity— 110 Kg/mm². The “2” refers to the year of manufacture, 2000.)

It’s all relative.

It is important to realize that there are no industry-standard definitions for words like “hard”, “soft”, “semi-soft”, and “very soft”. Each manufacturer’s hardness descriptions indicate only the relative softness of the various models offered by the same manufacturer, not the softness as rated on an industry-standard scale. That means that what one manufacturer characterizes as a “soft” boule might be called “semi-soft” or “very soft” by a different manufacturer.

What hardness should I choose?

For your first set of competition boules, we recommend “harder” (that is, more scratch-resistant) boules. Harder boules tend to be cheaper than softer boules, and they are less likely to develop gouges and burrs if you play on surfaces with hard, sharp rocks. Until you become much more skillful, the hardness of your boules is not going to make any significant difference in how successful your lobs or your shooting throws are.

For advanced players, soft or semi-soft boules may be a bit easier to control — although frankly I don’t buy into that old wives tale about softer boules being more likely to produce carreaux. Expensive boules with advanced anti-rebound technology are appropriate only for very advanced players, or players with money to burn.

[Update: 2019-07-11]
Obut changes “Durete+” to “Amorti+”
Raymond Ager points out that Obut has recently replaced the designation “Durete+” with “Amorti+“. The French word “amortir” means “to cushion” or “to dampen”, as in: to dampen a vibration. In tennis, “amorti” refers to a drop shot, a soft shot that just clears the net and then dies. You can see both connotations (cushioning of vibration, support for a drop shot) in Obut’s description of its Amorti+ boules.

An exclusive new technology from Obut!
Amorti+ boules are manufactured using high internal-strength steels and a unique quenching process that allows them to strongly surpress the effects of internal vibrations in the boule. An Amorti+ boule therefore possesses an extraordinary dampening capacity, notably on hard ground during a plombé (high lob) and a minimal rebound while shooting plein fer.

[Update: 2023-02-04]
An objective measure of rebound
Raymond Ager recently expressed a wish that there was some sort of standard, objective test that could measure a boule’s bounce and rebound under controlled conditions. “I think MS boules are great boules, but do the «internal stripes» REALLY give an advantage?” he asks.

I agree. In fact, I’m a bit surprised that some group hasn’t already developed a standard procedure to measure a boule’s rebound. It wouldn’t be difficult to do. Rebound could be measured in the number of millimeters that a boule of a specified weight and diameter rebounds when dropped from a specified height onto a plate of steel of a specified hardness. You could do it simply by dropping boules onto an anvil! 🙂

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