La Pavoni boiler question - Page 3

A haven dedicated to manual espresso machine aficionados.
User avatar
drgary
Team HB
Posts: 14348
Joined: 14 years ago

#21: Post by drgary »

I think you're asking a good question. Do you have slime that looks like that in your house plumbing? We have some slime in one of our pipes and have been told that chlorine bleach will clean it. I'm not suggesting you use something toxic like that in your espresso machine, but I wonder what will dissolve it if you scrape some out. Does detergent work? Chlorine? Vinegar?
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!

User avatar
drgary
Team HB
Posts: 14348
Joined: 14 years ago

#22: Post by drgary »

Here's some info from the late Dr. Pavlis on cupric oxide.
rpavlis wrote: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!
rpavlis wrote: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.
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!

toolate (original poster)
Posts: 281
Joined: 7 years ago

#23: Post by toolate (original poster) »

The black color sounds like CuO...
i know that is supposed to feel slimy but i never saw anyone describe it as actual SLIME.

User avatar
drgary
Team HB
Posts: 14348
Joined: 14 years ago

#24: Post by drgary replying to toolate »

I'm not sure about the slimy part, which is why I suggest removing it, using water that won't easily scale, emptying the boiler often and seeing if it returns as something slimy. It could be a combination of something organic and CuO. If you let us know what dissolves it that will be a clue. The important part is getting your boiler to the point you're satisfied you're brewing with clean water and also not overdoing descaling to where you've got green water because you're stripping copper out of your boiler.
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!

Post Reply