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KatyE 06-27-2010 10:13 PM

Tub cleaning last resort
Hi all. I have a very old cast iron tub on which the finish (porcelain?) is worn off in one spot that has been dripped on for years, and worn flat all over. Because it's lost all shine, the stains from rust and soap scum just won't come off. I've tried all kinds of cleaners and scrubbing. I've even dumped a whole gallon of bleach into the tub full of hot water and let it soak for hours. No good.

However, a few months ago, we had a slow drain. I poured a foaming drain cleaner into the drain, and the foam kind of built up on the tub around it. Lo and behold, it was pure white where the drain cleaner had foamed.

So, I'm wondering if it's a good idea to pour drain cleaner all over the tub and let it sit for a while, then rinse it off. Or would I be doing more harm than good? This tub's days are numbered, as we're hoping to renovate in the next few years, and I'm tired of looking at gray stains everywhere. But.....I don't want to do anything stupid. So what do you think? Is it a good plan? :help:

Nestor_Kelebay 06-28-2010 09:44 AM

When I saw the title of this post, I thought it was about cleaning soap scum off a tub, and was going to tell you that I use oven cleaner to clean the soap scum out of dirty bathtubs all the time.

You see, oven cleaner is normally sodium hydroxide, NaOH, whereas drain cleaner is typically potassium hydroxide, KOH. Both work by the dual action of:
a) converting grease and fat into soap through a chemical reaction called "saponification" and
b) dissolving hair (which is the most common cause of bathroom drain clogs).

You can learn more about saponification at the Soap and Detergent Manufacturer's
Association web site at: w w
If that doesn't work, go to w w and you should be redirected to the right web site.

You see, soap is made by combining an animal fat or plant oil (think Palm oil and Olive oil, after which the Palmolive company was named) with a strong alkali like sodium hydroxide (NaOH). What happens is that a sodium ion from the lye attaches itself to the end of a fatty acid chain from the fat or oil, so that you have a long chained hydrocarbon molecule with a Na ion at one end, which is a "soap" molecule.

It's the sodium ion at one end of the soap molecule makes soap soluble in water.

However, when soap is put in hard water with Ca++ and Mg++ ions, you can have one Ca++ ion react with two soap molecules to form soap scum, like this:

Two soap molecules:
long hydrocarbon chain-carboxyl group - Na+ ion (big space here) Na+ ion - carboxyl group - long hydrocarbon chain

form one soap scum molecule:
long hydrocarbon chain - carboxyl group - Calcium++ ion - carboxyl group - long hydrocarbon chain
and two sodium ions that float away.

and, that soap scum molecule is not soluble in water, but precipitates out of the water to form a soap ring around your tub.

By cleaning the bathtub with oven cleaner (lye or sodium hydroxide, NaOH), you provide additional free sodium ions which drives the reaction in the opposite direction; basically breaking down the soap scum to convert it back into soap again, which is easy to remove because it's soluble in water.

I don't think it's a good idea to pour a bottle of drain cleaner into your tub because it may cause damage to the drain piping and P-traps the drain cleaner could sit in. However, what you could do is let the tub dry fully, and then apply oven cleaner or drain cleaner to the tub with a sponge or paper towel. Remember that both oven cleaner and drain cleaner will also attack the oil on your skin (the stuff that leaves fingerprints) and the fat in your skin as well, converting it into soap too. (That's why your fingers feel slippery when you use oven cleaner on unprotected hands; the oven cleaner has converted the oils and fat in/on your skin into soap.) So, wear protective gloves so that you don't burn your skin with the stuff. You only need a little bit of oven cleaner applied to the soap scum, so a whole bottle of drain or oven cleaner should be considered a "lifetime supply".

That's all you need to convert the soap scum back into soap. Keep the plug in the tub all the time you're cleaning, and then put some water in the tub to greatly dilute the oven or drain cleaner before rinsing out the tub and draining it. You want to be sure that the drain cleaner is well diluted and that you wash all of the caustic drain cleaner out of the drain piping of your house.

The Soap and Detergent Manufacturer's Association web site has changed since they combined with the American Cleaning Association. I will phone them at (202) 662-2516 to see where the educational materials are on the new web site. I can't seem to find them.

KatyE 06-28-2010 01:51 PM

Wow. Good information. Thank you so much! I think I have oven cleaner around here, but I'm not positive, since we have had a self-cleaning oven for a few years now. If I do, I'll try that first. Thanks!

Nestor_Kelebay 06-28-2010 04:18 PM

Here is how to find that educational segment on the manufacture of soap from the American Cleaning Institute's web site:

Go to
Click on the "Clean Living" link
Click on the "At School" link
in the list of topics, click on "Soaps and Detergents"
in the resulting graphic, click on "Chemistry"

PS: On that web page the way they've depicted a soap scum molecule is wrong. That is, in this diagram:

That "film" molecule should have a green "hydrocarbon chain" on each end, not a yellow alkali group.

Someone was asleep at the switch.

Also, a better drawing of a typical animal fat or vegetable oil molecule (a "triglyceride") can be found on this web page:

The H2C-HC-CH2 on the extreme left is the glyceride, which forms a glycerine molecule when you add three OH- ions to it. (those ions coming from the NaOH or KOH that dissolve in water)

Those three -O-(C=O)- groups just to the right of the glyceride are three carboxylic acid groups, and

this business: /\/\/\/\/\/\/\ are the long hydrocarbon chains. There's supposed to be a carbon atom at each point where two lines meet, and you're supposed to image two hydrogen atoms bonded to each of those carbon atoms.

So, the glyceride is the crimson thing in the Cleaning Institute web page, the orange upside down hearts are the carboxylic acid groups, and the green pointed rectangles are the long hydrocarbon chains.

KatyE 06-29-2010 08:21 AM

Oh, my! What a difference the oven cleaner made! It's enough that I wish I had "before" pictures so that I could compare it.
Now, this tub is a hard case, so there are still some light stains, but it's ever so much better than it has been after cleaning it for the last several years. Thank you!!!!!!!!!
I have to admit, I went through more of the oven cleaner than you said to use. I closed the drain, sprayed it on as I would if I was cleaning the oven, and let it sit for ten minutes. Then I scrubbed it with a scouring pad, rinsed it with the shower, let the tub fill up a few inches, and let the drain go. As soon as the water drained out, I rinsed some more. It made such a difference that I let it dry and did it all over again, this time letting it sit for several hours. When I was done rinsing, I let the tub continue to run down the drain for about 5 minutes, to make sure the drain was clear.
I can't believe how much better it looks. I'm thrilled that I'll no longer have to be "icked" by the tub. Thank you so much!

granite-girl 06-29-2010 12:28 PM

That is good information Nestor. I'm always wowed by your knowledge. Seriously man, are you related to Einstein or something ?
Is that drain cleaner safe for the environment going into the drain system, then eventually into wherever it goes ? Into a septic tank then slowly leaked into the ground (in my situation)

Nestor_Kelebay 06-29-2010 09:00 PM

Granite Girl:

No, I have a scientific background (I graduated as a mechanical engineer, and then pursued a masters in chemical engineering, but never completed it). As very much of the work I do in my building involves cleaning, I took an interest in the chemistry of cleaning a long time ago in the hopes it would help me do a better job. That "saponification" reaction that converts oil into soap is one of only several chemical reactions that are used to clean things. But, it's an important reaction because oven cleaners use that reaction to convert baked on grease into soap, and drain cleaners use it to convert cooking oil into soap.

No, it's not a good idea to put drain cleaner down your drain piping because it's highly caustic and could cause copper and iron drain piping to corrode. You can, however, use it sparingly as I recommended to the original poster. Also, if you're on a septic system as you are, you might want to add a little vinegar to the water you use to rinse out the tub before pulling the tub drain plug. Oven cleaner will be basic, and vinegar is acidic, so they will neutralize each other, and that shouldn't be as bad for the beneficial bacteria growing in your septic tank. You should be able to buy a pH testing kit at any pet store (for aquariums), any pool/spa supply store or pharmacy to find out how much vinegar to add to the rinse water to neutralize it. A neutral pH is "7".

Did you know that our word for "soap" comes from the name of Mount Sopa outside of Rome in Italy. In Roman times, Mount Sopa was the place to go if you needed a favour from one of the many Gods. You would take along an animal with you, (a bird for a small favour, a pig or goat for a big favour) make your plea to the God, kill the animal as a sacrifice and burn it's body so that it would travel up to the God. Roman women noticed that washing clothes on the streams that ran down Mount Sopa after a rain got them cleaner than on any other mountain or at any other time. And, it's believed that the fat dripping off the burning animal carcasses combined with the ashes from the sacrificial fires to make a crude form of soap. That soap was then washed off the mountain by the rains and collected in the streams that Roman women did their laundry in. So, Roman women discovered that something about the water that flowed off Mount Sopa after a rain got clothes cleaner, and now we believe it was because of the formation of soap in the ashes of the sacrificial fires.

Anyhow, thanks for taking the time to read my posts.

Nestor_Kelebay 06-30-2010 09:50 AM

Granite Girl:

Last night it occured to me that you and KatyE might benefit by knowing how to avoid soap scum formation in the first place rather than just knowing how to convert that scum back into soap. And, since many others in here are probably in the same situation, I wrote a post called "The dirt on soap scum" in this forum you may want to read.

KatyE 06-30-2010 10:13 AM

My husband and I both had the same thought while reading your replies. You are the Alton Brown of cleaning. ;)

Nestor_Kelebay 06-30-2010 04:05 PM

Glad I was able to help. :)

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