Acid Test

dcarch7July 24, 2012

Thank you Lpink for the Piccalilli relish for Sienna 98. I will have a ton of green tomatoes at some point. Great recipe to use some up.

In the post, "---"Non-reactive" means not aluminum or cast iron, which will "react" with the vinegar to give an off taste to your relish. Enamel, non-stick or stainless steel are fine.---"

I seem to remember differently from Chemistry 101 about aluminum. Aluminum forms protective oxide immediately on exposed surfaces which prevents further chemical reaction when it comes to weak acid such as acetic acid. That's why aluminum containers are commonly used to store and transport the acid.

So I did an experiment.

Using an aluminum cup with a sheet of aluminum foil, I measured the weight (1.03 oz), and then I cooked the foil in pure vinegar for three hours. After, the cup and foil were let soaking for more than 36 hour.

I cleaned and dried the cup and foil and measured again, the weight was 1.03 oz.

I also examined the foil under a high power microscope. There were no visible pitting or corrosion on the shinny or mat finishes on the aluminum foil, compared with the untreated side of the foil.

We all know washing aluminum with mild alkali (basic) such as soap and dish washing liquid have no effect on aluminum, however, Easy Off oven cleaner will etch aluminum quickly. Aluminum will also react with strong acids such as hydrochloric acid.

Just being curious.


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I've seen aluminum foil develop holes when covering a saucy, bubbling lasagna. Here's what Reynolds has to say about that:

Why does aluminum foil sometimes melt and leave black specks on the food?
Occasionally when aluminum foil comes in contact with a different metal or a food that is highly salted or acidic, small pinholes are formed in the foil. This is a harmless reaction that does not affect the safety of the food. It is difficult to predict, but may occur under the following conditions:

1. When aluminum and a dissimilar metal are in contact in the presence of moisture, an electrolytic reaction may occur causing a breakdown of the aluminum. To avoid this use aluminum, glass, ceramic, plastic or paper containers. Do not cover sterling silver, silver plate, stainless steel or iron utensils with aluminum foil.

2. A similar reaction may occur when salt, vinegar, highly acidic foods or highly spiced foods come in contact with aluminum foil. The result of these reactions is a harmless aluminum salt. Some aluminum salts are used in medicines to treat stomach disorders. The food can be safely eaten; however, the aluminum salt particles can be removed from the food to improve the appearance of the food.

    Bookmark   July 24, 2012 at 9:00AM
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Interesting and valuable as usual. The microscopic examination and just the photo clearly support your point.

But the scale test is attempting to measure something of the wrong order of magnitude. The reactive change is probably in the miligram range, thousandths of a gram and your scale reads in the tenths of a gram. You would need a micro ballance and a drying oven, probably with vacuum.

    Bookmark   July 24, 2012 at 9:02AM
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I just remembered, too, several years ago borrowing my mom's Wagner aluminum dutch oven to make something. I don't recall what it was, but I'm guessing it was either a big batch of tomato sauce or chili. When I emptied the pot I saw it had discolored into a dark grey, almost black. It was a very disctinct line - below the surface level of the "whatever" it had turned color.

I'm sure it didn't affect taste any that I could tell, and I wasn't concerned about it from a safety aspect. But I was concerned about returning the pot to Mom looking like that! It took quite some elbow grease to remove the film.

    Bookmark   July 24, 2012 at 9:42AM
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I'm wondering if that was a marketing point for Revere Ware stainless steel cookware when it was introduced in 1939.

    Bookmark   July 24, 2012 at 11:41AM
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chas045, you are correct that measuring in oz may not have high enough resolution. It did occur to me that I should have set the scale to measure in grams. 1/100 of a gram is better than 1/100 of an oz.

However, the visual inspection with the microscope convinced me that there was no corrosion on the aluminum.

Optically, light wavelengths are not far from atom sizes. The change in light on a reflective surface can be effected by thickness of a few atoms. The anti-reflection coating on a camera lens makes the lens reflection bluish. A few atoms of oil (oil slick) on a puddle of water make the reflection of light in rainbow colors. The microscope inspection of the aluminum foil showed no difference at all on the tested surfaces.

Like all metals, including stainless steel, can be subjected to corrosion when two dissimilar metals are in an electrolytic solution, depending on their respective locations on the Periodic Chart. That�s what happens a lot when you see discoloration or pitting on aluminum.

It is a scientific mystery that, aluminum being the third most abundant substance on earth, our body has not developed much use for it. There are not too many aluminum compounds which are toxic to the body (as I remember), because of that, sometimes I think by reacting to impurities, aluminum can render some toxic substance harmless.


    Bookmark   July 24, 2012 at 5:59PM
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Stumpy you were not wrong...aluminum is one of the most reactive of reactive that it does not occur in a pure form in nature, we have to use electric current to extract it form the ore.
Exposed to air, aluminum quickly develops a dull gray color which is aluminum oxide. There is something in potato skins ( unfortunately I forget what....and am too lazy to Google it) which caused aluminum to turn black....but cooking something acedic like tomatoes will brighten that sucker right up! will a solution of cream of tartar ( which is tartaric acid).
I have several times, before I got smart ancd used plastic wrap, covered a casserole containing tomatos with foil, in a glass dish, with foil and refrigerated it...only to find those black holes you mention.....eww ick. It has nothing to do with electrolis because it was a glass pan, and the holes happened in the middle of the dish.

and, FOAS...I suspect it was potatoes you cooked in your mother's club aluminum....

    Bookmark   July 24, 2012 at 7:32PM
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Well now, Linda, you've stumped Stumpy even further. I know it turned black - not bright, because I would have welcomed bright! And I have a hard time thinking it was potatoes because I rarely even cook potatoes, let alone in bulk such that I'd have to borrow a big pot. Hmmm. Next time I head over there I'll see if there are still traces of that discoloration and take pictures if there are.

    Bookmark   July 24, 2012 at 7:47PM
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Could it have been a roast....with potatoes?

    Bookmark   July 24, 2012 at 9:25PM
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So what is the deal with the older version of anodized calphalon going from the lovely grey non-stick to bright silver when something tomato based is cooked in it? I remember ruining a brand new calphalon pot making spaghetti sauce and decided then and there never to cook anything acid based in aluminum cookware again. No amount of tin foil in vinegar (and what is "pure" vinegar anyway?) is going to convince me otherwise.


    Bookmark   July 24, 2012 at 9:49PM
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Vinegar you find for use in the kitchen is can buy 20% for agricultural killing weeds, but that is considered a hazardous material....don't know what hoops you would have to jump through to get it.
Presumably pure vinegar is 100%....and then it gets called acetic acid.

No idea why your calphalon turned "non-calphalon"...LOL! I have a couple I am very careful not to use for tomatoes or kraut or sweet and sour red cabbage.

I have my mother in law's wonderful old heavy wearever aluminum pot...I guess it's an open canner. It's very badly pitted, and I will guarantee it has never been in a dishwasher nor had anything alkaline cooked in it.

    Bookmark   July 24, 2012 at 10:34PM
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