Appendix II, 5a states that:
| The face of the club must be hard and rigid and must not impart significantly more or less spin to the ball than a standard steel face (some exceptions may be made for putters). Except for such markings listed below, the club face must be smooth and must not have any degree of concavity.
If claims of excessive spin are made by the manufacturer, or if there is strong supporting evidence of excessive spin, then the club would be deemed to be non-conforming.
The “hardness” Rule is particularly relevant to putters which have a urethane or other “soft” material inset in the face.
The measure for hardness is made using a durometer, and The R&A’s interpretation of the Rule is that a putter face must be no less than 85 on a Shore A scale durometer. This number was based on the hardness of a Surlyn ball. A simple measure of hardness in the field would be to use a fingernail. If a fingernail leaves a significant imprint in the face of a club, it is possible that the material does not satisfy the "hard and rigid" requirement. The face of a wood or iron club must be substantially harder than a putter face, i.e. no less than 75 on the Shore D scale.
In the field, "rigidity" is interpreted to mean that the face should not have any visible signs of movement or flex when manual pressure is exerted.
Where there is an inset in the face of the club, it should be flush with the rest of the face so that the face can still be considered smooth with no concavity. As this is sometimes difficult to achieve consistently and there will be manufacturing tolerances, we do allow an inset to be up to 0.006 inches (0.15mm) proud of the rest of the face and 0.004 inches (0.1mm) sunk.