The piano is a fragile musical instrument and it needs regular maintenance.  The hammer and the felt which covers it are two parts of the mechanism that require special attention. The felt can deteriorate due to many reasons and this leads to a change in the quality of the sound as well as the touch of the pianist.  Due to this natural or mechanical damage, the head of the hammer, as well as its weight, shape, thickness and hardness are modified.  Each of these elements has a harmful effect on the variety of tones of the piano : a loss of weight of the hammer due to the reduction of the volume of felt gives a sound that is less generous and causes the loss in the nuances.; a larger surface in contact with the strings diminishes the richness of the resonance;a reduction in the thickness of the felt brings the strings closer to wood head of the hammer causing the sound to become harder;an excessive hardness of the felt causes a metallic sound. The different causes of the deterioration of the hammer and the felt are: the natural aging and an intensive or frequent use of the piano cause an excessive compacting of the felt the environment of the piano : with humidity the wood and felt absorb water and swells, the felt  can become moldy.  In contrast when the air is too dry, the felt loses its elasticity.Insects – mites in particular attack the wool of the feltrodents