Can you leave a kneaded eraser out?

Kneaded erasers are a popular type of eraser used by artists, students, and others for erasing pencil and charcoal marks. Unlike traditional pink erasers, kneaded erasers can be molded and shaped, allowing you to erase very precisely. This makes them ideal for detailed artwork. However, kneaded erasers do have a sticky, gummy texture that brings up the question – can you leave them out or will they dry out?

The quick answer is yes, you can leave a kneaded eraser out for short periods of time. However, it’s best to store it in an airtight container or plastic bag when not in use. Over time, extended exposure to air can cause kneaded erasers to dry out and become hard or crumbly. Storing them properly will help extend their usable life.

How Do Kneaded Erasers Work?

To understand why air exposure is an issue, it helps to first look at what kneaded erasers are made of. There are a few key ingredients:

  • Plastic polymers – This gives the eraser its soft, malleable texture
  • Filler materials – Finely ground rubber or vinyl particles are commonly used
  • Pigments – For color, such as white or gray
  • Sulfur – Added to help vulcanize and bind the rubber particles
  • When these ingredients are mixed and heated, they bind together into the soft, stretchy material we know as a kneaded eraser. The sulfur is important, as it reacts with the polymer chains and forms crosslinks that create elasticity.

    Why Air Exposure Is an Issue

    Over time, the sulfur bonds in a kneaded eraser will start to break down through a process called reversion. This causes the polymers to become less crosslinked, making the eraser stiff and crumbly.

    Exposure to air speeds up this process. Oxygen in the air reacts with the sulfur bonds, essentially oxidizing them. This degradation is accelerated at higher temperatures. While a kneaded eraser left out overnight won’t change much, prolonged heat and air exposure over weeks or months will gradually make the eraser dry and unusable.

    Storing kneaded erasers in an airtight environment helps slow reversion by limiting air exposure. It also keeps them at cool room temperatures. This extends the usable life significantly.

    Best Practices for Storing Kneaded Erasers

    Based on the air sensitivity of kneaded erasers, here are some tips for proper storage when not in use:

    • Keep them in a sealed plastic bag or airtight plastic container. This limits air exposure.
    • Avoid storing in hot places like a car on a summer day. Heat speeds deterioration.
    • If the eraser becomes dried out, try reviving it by kneading and stretching it to restore elasticity.
    • Don’t store near pungent foods like onions. Kneaded erasers are porous and can absorb odors.
    • Check on erasers every few months and discard if they don’t re-soften when kneaded.

    With proper storage between uses, a high-quality kneaded eraser should last for many months. Some manufacturers claim their erasers last over 5 years with proper care.

    How Long Can You Leave a Kneaded Eraser Out?

    There is no exact time limit for how long you can leave a kneaded eraser out before it starts to deteriorate. It depends on factors like:

    • Temperature – Warmer environments increase deterioration
    • Humidity – Dry environments cause faster moisture loss
    • Air circulation – Breezy areas increase oxidation
    • Eraser quality – Cheap erasers may dry out faster

    As a general guideline:

    • Overnight or for a day – Little effect, can be left out with minimal damage
    • 1-2 weeks – May start drying if in hot, breezy spot
    • 1 month – Increased risk of drying and damage over time
    • 2+ months – Expect eraser to be fully dried out if left exposed

    For minimal risk of deterioration, put the eraser away in an airtight container after each use. Don’t leave it out for extended periods if possible. If you do misplace an eraser, check it over before use. If any part feels hard or stiff, that section has dried and should be discarded. With a quality eraser and proper care, it can last many years.

    Signs Your Kneaded Eraser is Drying Out

    Here are some signs that a kneaded eraser may be starting to dry out from excessive air exposure:

    • Hard, stiff sections or small hard bits inside
    • Cracks or fissures on the surface
    • Brittleness when trying to knead it
    • Loss of flexibility and inability to shape it
    • Grayish-white color instead of bright white
    • Crumbles easily and sheds rubber pieces
    • No longer able to remove pencil marks cleanly

    If you notice any combination of these signs, the eraser is past its prime. Any hardened sections need to be cut off and the rest should be discarded. Leaving damaged erasers out will only worsen their condition.

    Storing Other Eraser Types

    Kneaded erasers aren’t the only kind that can be affected by improper storage. Here are tips for other common eraser types:

    Pencil Erasers

    The soft vinyl erasers on pencils can also harden and shrink when left out. Minimize this by:

    • Storing pencils in a case or pencil box, not loose on a desk
    • Keeping them out of extreme heat or sunlight
    • Storing tip-down in a cup or holder

    Plastic Eraser Blocks

    These solid blocks of vinyl eraser material hold up better than kneaded varieties when left out. However, they can still dry out over time, becoming hard and unusable. Keep them in pencil cases or boxes between uses.

    Chalkboard Erasers

    The felt material on these erasers can harden if left chalk-covered. Rinse under water to clean after each use. Allow to fully dry before storing.

    Reviving Dried Out Kneaded Erasers

    If your kneaded eraser shows early signs of drying, you may be able to revive it and extend its life. Here are some methods to rehydrate and soften the eraser:

    • Knead vigorously – This can help break down hard sections and evenly distribute moisture
    • Microwave 5-10 seconds – Brief heating softens eraser
    • Soak in water – Hydrates eraser, then dry before use
    • Seal in bag with damp paper towel – Creates a humid environment
    • Use eraser cleaner – Special sprays help revitalize erasers

    These revival methods work best on erasers that are just starting to dry. If sections are hardened and cracked, those areas won’t soften again. Remove any hard bits and revive the rest of the eraser promptly when you notice early signs of damage before it worsens. With quick action, you can often extend the usable life of a kneaded eraser.

    When to Discard a Kneaded Eraser

    Here are signs that a kneaded eraser is beyond reviving and needs to be discarded:

    • Multiple stiff, hardened areas
    • Severely dried surface that doesn’t soften when kneaded
    • Cracked or brittle texture
    • No longer erases pencil marks cleanly
    • Has absorbed odors that don’t dissipate
    • Shows mold or mildew growth

    At this degraded state, the eraser is no longer functional. Bits may flake off and crumble, making a mess. Any mold is unhealthy. Toss the eraser and replace with a fresh one.

    Buying Quality Kneaded Erasers

    Not all kneaded erasers are created equal when it comes to longevity. Higher quality erasers made from premium ingredients will last longer than cheap varieties. Here’s what to look for:

    • Smooth, supple texture – Avoid gritty or stiff erasers
    • Bright white color – Grays/browns indicate lower quality
    • Minimal sulfur smell – Excess sulfur causes faster deterioration
    • Trusted art brands – Known manufacturers like Prismacolor or Faber-Castell

    Artist erasers sold in art supply stores typically have better quality control than generic school erasers. With proper storage and care, premium erasers should provide many years of service.

    Storing Unused Kneaded Erasers

    The storage guidance above applies to erasers currently in use. But what about spare, unused kneaded erasers? To maximize their shelf life:

    • Keep in original packaging until needed
    • Store in cool, dry location away from direct light
    • Avoid temperature extremes like hot attics or garages
    • Use FIFO (first in, first out) inventory management
    • Write date opened on eraser when packaging is removed

    With optimal storage conditions, an unopened kneaded eraser can last 5+ years before use. Once opened, use the eraser within a few years and replace with a fresh one when worn out. Proper inventory rotation helps reduce waste.

    Frequently Asked Questions

    Here are answers to some common questions about kneaded eraser storage:

    Can you freeze kneaded erasers to extend life?

    Freezing is not recommended. The moisture and temperature change from freezing can damage the eraser. Simply storing at room temperature out of sunlight is best.

    How should you dispose of worn out kneaded erasers?

    They can be tossed in the regular garbage. There’s no need to dispose of them in any special way.

    Can dried out erasers be hazardous if bits are inhaled?

    While not toxic, it’s best to avoid inhaling rubber eraser bits. Discard severely dried erasers that are cracking or crumbling.

    Should you store kneaded erasers in plastic or paper bags?

    Plastic bags provide an airtight seal and prevent drying. Paper bags allow more air circulation. For longest life, use plastic.

    Can you extend eraser life by refrigerating?

    Refrigeration can help extend life slightly, but room temperature storage in an airtight container works nearly as well. The temperature change of refrigeration can also cause condensation issues.


    Kneaded erasers are a versatile tool for artists and students when properly maintained. While they can be left out for short time periods, it’s best to store them in an airtight container between uses. Prolonged air exposure will cause drying and deterioration over weeks or months. Check your erasers periodically and discard any that show signs of damage. With quality erasers and good storage practices, they’ll provide years of service. Use care when storing other types of erasers too. A little maintenance goes a long way in extending the life of this useful tool!

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