When enough dust, dried spit and other air-born contaminants have settled permanently on a typically 6µ thin diaphragm, there comes a point when the weight of that debris will affect the transient response and high frequency extension of what the capsule can transmit mechanically, to be then converted by electronic means: too much weight on the diaphragm simply slows down its movement.
Another issue caused by capsule contamination is break-down of capacitance.
A condenser microphone capsule consists of a diaphragm (or two, in case of multi-pattern mics) and a backplate (or two), against which the diaphragm is mounted, at incredibly close distance (usually around 5-7µ) but incredibly high isolation resistance (ten-thousand million (10gig) ohms or more). Such close distance and high isolation between the two capacitor plates makes it imperative to keep both components meticulously clean.
If the isolation resistance between the two plates is reduced, through dirt accumulation caused by spit, and aided by the electrostatic attraction of airborne particles to the charged capsule, the capacitance created by the two capacitor plates (condenser is another word for capacitance), is suddenly reduced or collapses completely.
This capacitance collapse will render the capsule inoperative, i.e. sound will fade in and out, or lots of discharge sounds, from thunder to whistling noises ensue. That process of deterioration is accelerated when moisture or humidity is present: moisture helps to conduct electricity between individual dirt particles, and thus forms a conductive path. And as soon as conductivity between diaphragm and backplate is created, the capacitance between the plates discharges, and renders the capsule inoperative. Some capsule designs are structurally more resistant than others to that phenomenon, but ultimately, all conventional condenser mic capsules are prone to be affected by dirt accumulation.
To restore the capsule to its original high-impedance, uncontaminated state, all dirt and moisture must be removed, to regain the original super-high impedance between the relevant capsule parts.
I have always been reluctant to self-promote on my forum, and continue to try to resist it to the utmost. But I am passionate about correct capsule cleaning, and its many amateurish, destructive perpetrators (including the unconscionable application of distilled water with a brush, promoted at one time by a major mic manufacturer) continue to irritate me. The frequent mentioning of distilled water as cleaning agent is fraught with peril: contaminants floated that way can easily penetrate the minute gap between the plates and permanently settle there. ANY mechanical contact with the gold sputtering of the diaphragm has further the disadvantage of scraping dirt particles right into the layer of gold.
Capsule cleaning is an art, based on science, and I continue to warn mic owners to not try to use mechanical means in attempting to clean a diaphragm, a backplate, and its associated high-impedance components. The gold laid on most large diaphragm capsules is but a few angstroms thin, and, as mentioned, any mechanical contact with that ultra-thin layer will remove precious amounts of the conductivity the gold facilitates. There are methods to restore a capsule back to factory specifications that do not involved mechanical rubbing. I have developed such method, and others may as well. It is vitally important that any microphone owner who suspects capsule contamination interviews service providers as to the method they use to remove the contaminants. In case of doubt, I recommend to buy a new or used, but clean, capsule, rather than turn the defective capsule over to an amateurish cleaning attempt.
My aversion to "reskinning" (re-diaphragming) a contaminated capsule will be fodder for another sticky.