[schmidling]

For more than 20 years, we have been tumbling aluminum castings with soap, vinegar, water and stainless steel screws.  Depending on the soap we use, we get anything from a mirror finish to grungy, greasy, black yuck.

I have had to change soap brands over the years because the chemistry of the soap changed or the brand disappeared from the market.  We have been using Dawn dish soap for many years with no problems.

Recently I have been making soap as a hobby and naturally thought it would be fun to make my own soap for the tumbler.

The ones that work, do so only with the vinegar added so I presumed it had something to do with pH.  Dawn is about pH8 and my home made liquid soap is about pH10.  The pH of the solution with soap and vinegar is around pH4.  Knowing that aluminum reacts with alkali is the reason I tried the vinegar in the first place.

Using my soap produces the usual non-Dawn grunge.

I added enough citric acid to the soap to get a pH of 7 and used this with the vinegar and water and still got the grunge.  Ru-tumbling the grungy parts in Dawn made them clean and shiny as usual.

Any idea what is going on here?

Jack

[Arkcon]

Soaps, of various types, are sodium or potassium salts of fatty acids.  They are soluble in water, and good cleaning agents.  Other metal ions can replace the sodium and potassium, these are still metal salts of fatty acids, but many aren't soluble.  Aluminum "soaps" -- the aluminum salts of fatty acids a very sticky and gloopy -- its mixed with gasoline to make napalm.  So soap is a poor choice as a surfactant in polishing aluminum.  Dawn is a detergent, its not a metal salt of anything, and is likely not as reactive to aluminum.

I don't know how fast bulk aluminum reacts with soft soaps, and I don't know whats wrong with other detergents that are likely also detergents like Dawn.  So I can't fix all the problems you're having.  But in a practical way, you should just use Dawn.  Or maybe purchase non-ionic or anionic laboratory grade detergents.

[schmidling]

But in a practical way, you should just use Dawn.  Or maybe purchase non-ionic or anionic laboratory grade detergents.

I spent my life being practical and I guess that's why we have been sticking with Dawn. 

We had a goose for dinner yesterday and I made a liter of soap out of some of the grease.  Not sure that is practical, fun or a waste of good goose grease.

It is nice to find out why something doesn't work and this forum is a good place to do just that.

Thanks,

Jack

[Arkcon]

And now you also know how to make (safe) half of napalm.  Should that application ever come up.

[curiouscat]

We had a goose for dinner yesterday and I made a liter of soap out of some of the grease. 

That made me smile! Good job!