<p>Short story: <strong>They are probably aluminum oxide</strong>, but almost certainly some kind of metal oxide. <strong>You don't need to take any action unless they're affecting the taste of your coffee</strong>, and it shouldn't be. You probably <em>can't</em> (or <em>shouldn't</em>) really permanently remove them even if you wanted to.</p>

<p>Longer story...</p>

<p>This is a <a href="http://www.bialetti.com/coffee/stovetop/moka-express-c-1_7_22.html" rel="nofollow noreferrer">classic Bialetti</a> model, and a classic Bialetti symptom. The vessel is probably made out of alumin[i]um alloy (though there are similar models that are made of stainless steel, your picture seems to be looking down into the bottom heating vessel, and it looks like the classic aluminum species). Unfortunately the symptoms you're seeing, from my perspective, is a classic <em>failure</em> of the Bialetti design. Aluminum tarnishes (oxidizes) somewhat readily, especially when it is hot and in contact with water. Correspondingly, the only way to use this thing is, indeed, to heat it with water.</p>

<p><strong>The vessel, especially, needs to be rinsed and <em>dried thoroughly</em> in The Right Way.</strong> The <a href="http://www.bialetti.com/coffee/stovetop/moka-express-c-1_7_22.html" rel="nofollow noreferrer">"care" page for Bialetti</a> (follow that link, then click on the "Care Use" tab; I can't figure a direct link) suggests to <em>dry thoroughly</em> and most tellingly "Do not reassemble the product until all parts are completely dry to avoid oxidation." Even if you're meticulously drying the thing, it's still going to be a little hot and a little damp; this is going to invite oxidation. </p>

<p>Standard ways of removing tarnish are even more damaging in the long term than just leaving them alone. Aluminum oxide generally protects the metal from further oxidation (unlike rust -- iron oxide -- which flakes off and <em>facilitates</em> further oxidation).</p>

<p>I have a stainless steel moka pot, and it's terribly tarnished on the outside from heating on a gas burner hundreds of times, but the inside is clean (and clean<em>able</em>) as the metaphorical "whistle".</p>

<p>For completeness: It's also possible that they are mineral deposits; <a href="https://coffee.stackexchange.com/a/1849">this answer</a> to the other question that you linked has some additional info about that possibility. It would come from a buildup ("scale") from using "hard" water, and more so when nearly boiling dry (also not a good idea); filtered or distilled water is much less likely to do this. However, I think it's unlikely that you'd see scale buildup over the span of a couple dozen uses.</p>

CC BY-SA 3.0
2016-10-23T12:26:44.830Z
Is it safe to still use/drink from?
Nick Grealy
CC BY-SA 4.0
2020-10-21T07:45:12.293Z
@NickGrealy edit: Aluminium Oxide does seem dangerous ["harmful if swallowed"](https://web.archive.org/web/20090214112832/http://www.sciencestuff.com/msds/C1181.html), so the question is whether this requires larger amounts, or whether aluminium oxide does dissolve into the coffee...
dasWesen
CC BY-SA 4.0
2020-10-21T07:50:35.810Z
I didn't find the "care" page, maybe it's the same as this one: https://www.bialetti.co.nz/blogs/making-great-coffee/tips-and-care
dasWesen
CC BY-SA 3.0
2015-08-08T03:01:31.993Z
2016-02-17T17:54:41.707Z
2017-04-13T12:51:53.733Z
hoc_age
<p>I've got these spots on the inside the bottom section of my Bialetti Moka Express:</p>

<p><a href="https://i.sstatic.net/Y21rR.jpg" rel="noreferrer"><img src="https://i.sstatic.net/Y21rR.jpg" alt="enter image description here"></a></p>

<p>They are placed pretty evenly around the ridgy part toward the edge of the bottom of the circle, with a few toward the middle. They are only on the bottom of the pot as far as I can tell, not the sides. They look sort of like black mold, but I gave the pot a gentle scrub with warm water and a mild soap and they didn't budge.</p>

<p>I don't normally use soap on my pot. Generally I make a cup of coffee, rinse all components, and allow them to air dry. Sometimes when I'm pressed for time, the just-used pot sits in the sink for a few hours without getting disassembled or rinsed. I have a hunch that that moisture may be contributing here. I use filtered water, so I don't think they are mineral deposits (unlike in <a href="https://coffee.stackexchange.com/questions/1847/moka-pot-buildup">this case</a>). The pot is pretty new. It's been used 1-3 times per day for about three weeks.</p>

<p>So what are these spots? If they aren't affecting the flavor of the result, do I need to worry about them? If so, how can they be removed?</p>

CC BY-SA 3.0
2015-08-07T17:44:50.697Z
Maybe heat discoloration if you are having a difficult time scrubbing it off? Try rubbing with a really fine powder like baking soda and see if it comes off, this is how i clean my stainless steel stove-top kettle.
rwyland
CC BY-SA 3.0
2015-08-07T22:39:17.167Z
Maybe rust perhaps?
Anthony Pham
<moka><cleaning>
What are these spots on the inside of my moka pot? Do I need to remove them?
hairboat
# What are these spots on the inside of my moka pot? Do I need to remove them?

(Asked by: username_0 on 2015-08-07)

I've got these spots on the inside the bottom section of my Bialetti Moka Express:

[![enter image description here](https://i.sstatic.net/Y21rR.jpg)](https://i.sstatic.net/Y21rR.jpg)

They are placed pretty evenly around the ridgy part toward the edge of the bottom of the circle, with a few toward the middle. They are only on the bottom of the pot as far as I can tell, not the sides. They look sort of like black mold, but I gave the pot a gentle scrub with warm water and a mild soap and they didn't budge.

I don't normally use soap on my pot. Generally I make a cup of coffee, rinse all components, and allow them to air dry. Sometimes when I'm pressed for time, the just-used pot sits in the sink for a few hours without getting disassembled or rinsed. I have a hunch that that moisture may be contributing here. I use filtered water, so I don't think they are mineral deposits (unlike in [this case](https://coffee.stackexchange.com/questions/1847/moka-pot-buildup)). The pot is pretty new. It's been used 1-3 times per day for about three weeks.

So what are these spots? If they aren't affecting the flavor of the result, do I need to worry about them? If so, how can they be removed?

**Tags:** moka, cleaning

> **Comments:**
> **username_1**: Maybe heat discoloration if you are having a difficult time scrubbing it off? Try rubbing with a really fine powder like baking soda and see if it comes off, this is how i clean my stainless steel stove-top kettle.  
> **username_2**: Maybe rust perhaps?  

---

## 1 answers

### Answer by username_3 on 2015-08-08. Score: 13 | Accepted answer

Short story: **They are probably aluminum oxide**, but almost certainly some kind of metal oxide. **You don't need to take any action unless they're affecting the taste of your coffee**, and it shouldn't be. You probably *can't* (or *shouldn't*) really permanently remove them even if you wanted to.

Longer story...

This is a [classic Bialetti](http://www.bialetti.com/coffee/stovetop/moka-express-c-1_7_22.html) model, and a classic Bialetti symptom. The vessel is probably made out of alumin[i]um alloy (though there are similar models that are made of stainless steel, your picture seems to be looking down into the bottom heating vessel, and it looks like the classic aluminum species). Unfortunately the symptoms you're seeing, from my perspective, is a classic *failure* of the Bialetti design. Aluminum tarnishes (oxidizes) somewhat readily, especially when it is hot and in contact with water. Correspondingly, the only way to use this thing is, indeed, to heat it with water.

**The vessel, especially, needs to be rinsed and *dried thoroughly* in The Right Way.** The ["care" page for Bialetti](http://www.bialetti.com/coffee/stovetop/moka-express-c-1_7_22.html) (follow that link, then click on the "Care Use" tab; I can't figure a direct link) suggests to *dry thoroughly* and most tellingly "Do not reassemble the product until all parts are completely dry to avoid oxidation." Even if you're meticulously drying the thing, it's still going to be a little hot and a little damp; this is going to invite oxidation.

Standard ways of removing tarnish are even more damaging in the long term than just leaving them alone. Aluminum oxide generally protects the metal from further oxidation (unlike rust -- iron oxide -- which flakes off and *facilitates* further oxidation).

I have a stainless steel moka pot, and it's terribly tarnished on the outside from heating on a gas burner hundreds of times, but the inside is clean (and clean*able*) as the metaphorical "whistle".

For completeness: It's also possible that they are mineral deposits; [this answer](https://coffee.stackexchange.com/a/1849) to the other question that you linked has some additional info about that possibility. It would come from a buildup ("scale") from using "hard" water, and more so when nearly boiling dry (also not a good idea); filtered or distilled water is much less likely to do this. However, I think it's unlikely that you'd see scale buildup over the span of a couple dozen uses.

> **Comments:**
> **username_4**: Is it safe to still use/drink from?  
> **username_5**: @NickGrealy edit: Aluminium Oxide does seem dangerous ["harmful if swallowed"](https://web.archive.org/web/20090214112832/http://www.sciencestuff.com/msds/C1181.html), so the question is whether this requires larger amounts, or whether aluminium oxide does dissolve into the coffee...  
> **username_5**: I didn't find the "care" page, maybe it's the same as this one: https://www.bialetti.co.nz/blogs/making-great-coffee/tips-and-care  

---

