I bought a brand new Europiccola 9 months ago.
The machine is descaled once a month using 50% white vinegar 50% water.
Yesterday, after descaling it, I noticed that in the inside of the boiler at the top half of it, there is some red dust. I tried to scratch it with my fingers, it left on my fingertip a very dark brown or even black mark, which is not easy to be removed, as if it had some kind of grease.

What is that ? Has any Europiccola owner noticed anything similar ?
Could that be any copper oxide ? Is that dangerous ? Any idea on how I can remove this ?

I am a retired chemistry professor, and I once worked for a copper mining company whilst on Sabbatical leave. The chemistry of copper is far more complex than most people realise. Moderately alkaline and strongly alkaline water results in a reaction sequence involving Cu+ and Cu2+ that can bring about very rapid corrosion, especially at elevated temperatures. (Such as the 116 degrees temperatures that obtain in an espresso machine!)

Strongly acidic water prevents any oxide coating from forming, and having the bare metal exposed to the solution can also bring about severe corrosion, especially in water with high chloride content because of the formation of chloride complexes, both with Cu2+ and Cu1+.

Thus both strongly acidic and strongly alkaline conditions are destructive to Cu.

I suspect the material you are seeing is a mixture of CuO and Cu2O. When you clean with acetic acid do you get a blue solution? Blue, of course, means you are slowly dissolving your copper boiler!!!

Cu2O is reddish, CuO is black.

Many people do not like the taste of espresso made with distilled water. I happen to prefer it that way! The presence of ions in water tends to reduce solubility of organics, so contrary to what some say, you get the best extraction with pure distilled water. Distilled water contains very few ions, so it has low electrical conductivity. This prevents electrolytic corrosion from dissimilar metals in contact with one another. It tends to become weakly acidic from dissolution of carbon dioxide, but on heating this is expelled anyway.

A neat chemical demonstration of the effect of ions on organic material solubility is to add solid potassium carbonate to strong rum or whiskey. When you get a certain amount of the K2CO3 dissolved, the ethanol and water actually separate! The ethanol will form a separate layer floating on the water!!!

From a corrosion standpoint it is highly preferable to use distilled water. The only reason to use tap water is because one prefers the slightly different flavour. The life expectancy of an espresso machine using only distilled water has to be dramatically higher than one using hard alkaline water!

Thanks a lot Robert for the reply,
I hope now you have the time to enjoy your life and excellent espresso shots!

Strongly acidic water..., especially in water with high chloride content because of the formation of chloride complexes, both with Cu2+ and Cu1+.

. Most probably the water in Athens has high chloride content, but it is not very hard. The water I use for the espresso machine is always coming from a Brita to reduce the chloride & the hardness.

When you clean with acetic acid do you get a blue solution? Blue, of course, means you are slowly dissolving your copper boiler!!!

I use only 50% water (from the Brita) and 50% white vinegar and very rarely citric acid (not with vinegar). I do not know if vinegar contains acetic acid.
The water coming out during the descaling does not have any color at all. It looks like tap water. I boiled some water in the machine. Got it out through the group, let it get cold and tried it. It tasted ok, very similar taste to the water Brita produces.

Cu2O is reddish, CuO is black.

In my case, the color is rather reddish, not black. I tried again scratching the boiler with my nails. Does not feel list "dust". It seems like the color of the boiler has changed.

From a corrosion standpoint it is highly preferable to use distilled water.

I have tried pure distilled water from a drugstore. My conclusions:
* Hard to find. Not every drugstore here has it available.
* A bit expensive, since I have to throw away a liter of water for every espresso I make. Distilled water can not be store because it does not contain chloride. Correct ? So after the coffee, I have to empty the machine and throw the water away.
* Distilled water if not stored properly can be contaminated. I do not know how supermarkets store it. Because of this I prefer to buy it from drugstores.
But I also prefer the taste of espresso made with distilled water. I know it is better for the Europiccola. I compromised with the Brita.


Is there a way I can get rid of this reddish color ? Do you think it is dangerous to use the machine ?

The coating of copper oxides actually protects the copper from further oxidation. If you are not seeing blue colour when you clean with dilute weak acids I think you are in very good shape. That means very little copper compounds are present, even with the dilute weak acids. If it were me, I would not hesitate to drink espresso from this machine.

White vinegar is usually about 1.5 M acetic acid. (M is moles/litre). If you dilute it with an equal volume of water you have 0.7 M acetic which is, to my mind, ideal. Citric acid is a solid dicarboxylic acid with similar ionisation constant. I do not see any reason to prefer one over the other. Both are metabolic intermediates and hence non toxic. When I first got my used brass Europiccola the former owner had used water with VERY high Ca. It was full of CaCO3. I put the identical solution you used in it and heated it up to about 50 degrees. I let it stand for about an hour and then drained it out. I repeated the operation. Then I rinsed it three times with tap water, and then once with distilled. There were no white deposits left at all. However the pressure switch still did not work correctly. On examining closely I noticed white deposits in the tube leading to the pressure switch. I mixed vinegar and water to get about 0.3 M acetic acid, closed the lid, and brought it up to pressure. I bled off the air to bring the internal temperature to about 116 degrees. I let it cool. I looked inside and saw bubbles coming from the pressure switch, so I heated it again. The pressure switch has worked perfectly since, and there are no deposits at all. (This was last May!) I use only distilled water.

Distilled water here costs about 25 cents/litre. Because it is inert and basically a pure compound, I only drain it about once a month, otherwise I just top it up. I run about 20 mL through before the first cup. When I finish I have a special cup that fits under the group. I fill it with water and lift the handle twice, to draw the tap water into the group and discharge it again. Then I pull up the handle and let about 20 mL of hot distilled water from the reservoir pass through into the cup. Then I take a towel and dry the base and run a cotton swab around the seal where the portafilter goes. I use perhaps only 100 to 125 mL of distilled water daily.

I look into the reservoir with a light each day to spot problems before they become big.

Thanks Robert,
From your post, I understand that I am causing the problem, because I am over descaling, and to make it worse, I heat up the machine at brew temperature, which makes the solution more drastic causing corrosion of the copper parts in the boiler.
I liked this technique you suggested to clean the group after brewing espresso. I did it this morning. Very effective.

Hi Robert,
I read that a mixture of salt with vinegar could remove the reddish color from the inside of the boiler. Should I try that ? What should be the proportion of salt, vinegar (and water ) ?
You said that this reddish color is not really a problem, but every time I look at it, I just do not like it.

The red colour is almost certainly Cu2O. It is insoluble in neutral water and serves as a barrier to the copper underneath. Both Cu2+ and Cu1+ form strong complexes with chloride ion. The Cu1+ complex is soluble and reacts VERY rapidly with air to produce the Cu2+ complex. Partly oxidised it forms a very dark brown solution.

I do not see any reason to remove the oxide coating. In the presence of strong enough acid and chloride ion it would dissolve immediately. Unfortunately there is a process that occurs when the Cu2+ chloride complex is in contact with copper--they react to dissolve some of the copper and reduce the Cu2+ to Cu1+.

Now the Cu1+ reacts with air, making more Cu2+ which reacts with more Cu making more Cu1+, and we have a cycle set up which will dissolve eventually all the metallic copper! This is, in fact, the basis of acid corrosion of copper. There is a somewhat similar cycle set up under basic conditions, but without involving Cl.

You certainly do not want the acidic salt solution to contact the boiler for long periods of time. Water for copper boilers should be near neutral pH. Chloride ion is bad, especially in acidic conditions.