dust is not wiped from the varnish and left for long enough, it will fuse with the varnish and become impossible to remove without damage. Cleaning the rosin off strings can make a striking difference to the sound. A common wine cork serves admirably, quietly scrubbing off the crust of rosin without damaging the winding of the string. A dry microfiber cloth is often recommended; it retains the dust well, but makes a penetrating squeaking noise. A cloth with a little rubbing alcohol is effective, if care is taken to protect the top of the violin from the slightest chance of stray droplets of alcohol touching the varnish. The use of alcohol is generally avoided, as it easily damages varnish in ways which may be difficult or impossible to restore. The tuning pegs may occasionally be treated with "peg dope" when they either slip too freely, causing the string to go flat or slack, or when they stick, making tuning difficult. Some violinists and luthiers use a small amount of ordinary blackboard chalk on pegs to cure slippage. "Peg drops" (a solution of rosin in alcohol) may occasionally be used to treat slipping pegs, but that is a temporary solution at best; quite often slipping pegs have shafts which are no longer smoothly conical and should be refitted or replaced. The violin will benefit from occasional checks by a technician, who will know if repairs need to be made. Violinists generally carry replacement sets of strings to have a spare available in case one breaks. Even before breaking, worn strings may begin to sound tired and dull and become "false" over time, producing an unreliable pitch. Another common problem with strings is unravelling of the metal winding. Strings may need replacement every two or three months with frequent use. The higher strings require replacement more frequently than the lower strings since they are lighter in construction to produce a higher sound– fortunately their lighter weight also means they cost less. The price of strings varies, and the quality of the strings strongly influences the timbre of the sound produced. A teacher can advise students how often to change strings, as it depends on how much and seriously one plays. For the bow, the only real maintenance is regular cleaning of the stick with a cloth (to remove rosin dust), re-hairing, and replacing the leather and silver wire wrapping. In the