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The water will soak down into the shaft, causing rust or rot. It could also cause the glue holding the bristles to loosen. It is safe to store brushes upright once they are completely dry. The intense heat from the hairdryer or flatiron will ruin the fibers—even if the fibers are natural, such as sable or camelhair. The brushes are a makeup brush are mush more fragile than the hair on your head. If you dry your brushes in an enclosed area, such as the bathroom, the bristles may not get enough air-flow, which might cause mildew. This will result in musty-smelling bristles. Ew! When your brushes are dry, store them either upright in a cup, or lay them down on their side. Do not store them pointing down, or the bristles will get crooked. If you want to carry your brushes in a bag, store them flat in a brush case or bag dedicated to your brushes. Before you set your makeup brush out to dry, or even between washes, disinfect your brush with a vinegar-water solution. Don't worry, the strong, vinegar smell will disappear once the bristles dry. Fill a small bowl or cup with two parts water and one part vinegar. Swirl your brush in the solution, but avoid wetting the part where the bristles meet the handle. Rinse the brush with clean water and set it out to dry.
Do not store brushes upright while drying them. Do not use a hairdryer or flatiron on your brush. Dry your brushes in a well-ventilated area. Store your brushes properly. Consider disinfecting your brush.