The Reason There Are Black Lines Inside Of A Ceramic Coffee Mug (And Is It Safe To Use?)

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One of the tell-tale signs that a mug is old in a roadside diner isn't always the cup's style, although that's a giveaway, too. It's the black lines that mar the cup's interior. However, truth be told, such a phenomenon doesn't just happen to those cheap restaurant mugs. The black scratch marks at the bottom of the cup can happen to any ceramic mug that has gotten a lot of use. Those marks are essentially the rubbings from the edges of your spoon scraping off the silverware and onto the interior of your mug. What a way to ruin your favorite mismatched mug collection. 

Aside from being unsightly, those marks can present a health risk — And the harm doesn't only come from your spoon scrapings. The ceramic in your mug may present hazards, as some types of ceramic contain metals, usually cadmium and lead. From the lead could arise (potentially) reproductive issues, developmental delays, and psychological challenges. Conversely, the cadmium wreaks havoc on the kidneys and digestive systems, creates flu-like symptoms, and in some cases, can lead to death if the poisoning from the metal is significant enough. 

Whether or not your mug is vulnerable to such marks comes down to the chemistry of it's glaze. Some glazes can be very susceptible to the effects of friction, depending on how soft the glaze is. Your spoon scraping against the sides of the ceramic just unearths those metals in the glaze. Suffice it to say that if you have marks in your mug, its glaze has some susceptibility to this phenomenon, so it's best to take steps to prevent it.

Most of the time, you won't have to toss your mug out if it has accumulated its share of battle scars from your spoon. You don't necessarily need to switch to stainless steel mugs, either. You can both take steps to prevent scratches from happening in the first place, and you can clean your cup to remove the scarring that it does have. First things first: To avoid getting scrape marks on your mugs, use a plastic or wooden stirrer, such as Gmark wood kayak paddle-shaped sticks, to stir your coffee. This prevents the marks before they can happen.

If your mugs are already marred up, not to worry. You'll need a solution of baking soda, water, and a dish sponge with a scratcher side. It takes about 1 tablespoon of baking soda and ½ teaspoon of water to make the cup-cleaning-miracle paste. You'll apply it to the insides of your spoon-marked mug and scrub. You may have to apply quite a bit of elbow grease if your mug sports a veritable highway of spoon track marks. In any case, you'll need to scrub the mug with the paste and rinse it until the marks are gone.

Here's a bonus: If your ceramic mugs have turned slightly brown in color due to coffee and tea stains, this trick works to rid you of those stains, too. Just apply the same technique to keep your mugs stain-free and ready to use. Talk about killing two types of stain at once and with just one type of scrub to boot!