Mascara becomes clumpy and dry months before an eyeshadow palette shows any change. The cause is the design of the tube rather than the quality of the formula.
How the wand and tube work together
A mascara tube is narrow and fitted with a wiper that scrapes excess product from the brush as it is withdrawn. That wiper also seals the opening between uses.
Every insertion and withdrawal of the wand displaces air. The tube effectively acts as a pump, drawing fresh air down into the product with each application.
By comparison, a pressed powder is exposed at the surface only, and a pump bottle admits almost no air at all. Mascara is the extreme case.
Why water in the formula matters
Most mascaras are emulsions of wax, pigment and water. The water carries the formula onto lashes and then evaporates, which is what sets the coating in place.
The same evaporation happens inside the tube, slowly, every time air is introduced. The remaining product becomes thicker and the pigment-to-liquid ratio shifts.
Waterproof formulas use solvents rather than water as the carrier, which is why they thicken on a different schedule and are harder to remove.
Why the eye area sets the safety limit
Water and warmth are the conditions bacteria need. A tube repeatedly reopened, warmed by a bathroom and touched to the lash line meets all of them.
The eye has less protective barrier than facial skin, and the conjunctiva is directly exposed. Contamination that would be harmless elsewhere can cause irritation or infection here.
Eye redness, swelling, discharge or pain after using an old product is a reason to stop and see an eye doctor rather than to switch brands.
What the label symbol is telling you
Cosmetics sold in the United States often carry a small open-jar symbol with a number and the letter M. That is the period after opening, in months.
The figure describes how long the formula is expected to remain stable once air has reached it, not how long the sealed package lasts on a shelf.
For mascara that figure is consistently the shortest of any category, which reflects the packaging mechanism rather than any weakness in the product itself.
Why adding liquid makes it worse
A common response to a drying tube is to add water or saline to loosen it. This dilutes the preservative system that was formulated for the original volume.
Preservatives are dosed to control microbial growth in a specific formula at a specific concentration. Thinning that formula lowers the protection without any visible sign.
The tube then looks usable again while being less protected than it was, which is the opposite of what the person adding water intended.