Thanks for your question, had been wondering about this for a while but had not taken the time to search. Here are a couple of excerpts with pretty good explanations.
“Body odor results from bacteria growing in nice warm water (i.e. sweat). You can control it by stopping the sweat and/or by restricting the bacteria. You might also not want the wetness itself.
Regular antiperspirant works by having an ingredient off a short FDA list, all of which are aluminum salts. When water from sweat mixes with one of these salts, their component chemicals break apart into “ions”, which are positively or negatively charged. The aluminum ions are positively charged and small, which apparently means that they can pass through skin cell membranes, where somehow they either cause the cell to puff up, or help form a physical plug, thereby squeezing off nearby sweat glands. No sweat also means no water for the bacteria, so no odor.
This effect is temporary, as capillaries serving the cell clean up the ions, making daily reapplication necessary for a continued effect. Theoretically the ions are eventually removed from the body in the urine, but research suggests that certain tissues (liver, kidney, brain, cartilage and bone marrow) selectively absorb it.
The FDA has recently required a new label on antiperspirants saying that people with impaired renal function (i.e. those who have trouble shedding environmental toxins) and children should not use these products. The industry is complaining due to lack of scientific evidence, but someone at the FDA seems to believe in “prudent avoidance”!
The crystal deodorants, on the other hand, typically contain potassium alum. This is the same alum that used to be used in pickles, and it is a powerful astringent. I believe it occurs naturally or can be manufactured. It has the chemical formula KAl(SO4)2•12(H2O), which includes potassium, aluminum, sulfur (as sulfate), and water.
Aluminum is apparently unusual in that it can be part of either a positively or a negatively charged clump of elements, and when alum reacts with water (“hydrolyzes”), it breaks up into its components, rearranges itself into different clumps, and the aluminum goes with a negatively charged (and large) clump. (Sorry about the hand-waving here, I haven't been able to find the actual chemical reaction.) Supposedly this means that the aluminum cannot pass through the skin cell wall, the apparent basis of the claim of safety.
Disturbingly, however, according to Material Data Safety sheets (MSDS's), when potassium alum hydrolyzes, another resulting chunk is a dilute solution of sulfuric acid (H2SO4). Sulfuric acid is, of course, corrosive, and is probably the reason for the “discontinue if rash forms” warning on the label. I wonder if it is this acid that provides an antibacterial action for these products.
I am still mystified as to why, if 30g of alum has been known to fatally poison adults, it is not more carefully labeled in its grocery store package (especially as it is reputed to be somewhat sweet to the taste.) You’d think people would also get ill from sulfuric acid if they ate too many pickles, although the residue after rinsing during the canning process is supposedly minute.
I am also suspicious of the unspecified “mineral salts” ingredients listed on the crystal deodorants. I speculate that there may be other salts similar in action to the approved aluminum ones, and that ions from these may similarly be absorbed through the skin cells. My own experience with the crystal deodorants has been that they do indeed control wetness, but perhaps they cannot call themselves "antiperspirants" because the active ingredient isn't on the FDA's list.
So what’s a person to do? My current guess is that if you must use something in addition to general hygiene and instead of odor-masking perfume, if you can use the crystal deodorant without noticeable skin reaction, it is probably safer than regular deodorant. Any trouble due to the sulfuric acid is likely to be external, immediate, and noticeable, rather than internal, cumulative, and undetectable. Now you just have to worry about those anonymous "mineral salts".
I did contact one of the crystal deodorant manufacturers, and the person who replied was friendly but not up on the chemistry.”
Excerpt taken from: http://www.chemicalforums.com/index.php?board=9;action=display;threadid=3080
“Our deodorant stones are made of potassium alum. It is a pure product made without the addition of chemicals, fragrances, oils or alcohol. The chemical formula for potassium alum is K2SO4Al2(SO4)324H20. Potassium alum is a colorless substance that forms octahedral or cubic crystals.
Bauxite is the ore from which alum is drawn. It is formed by the rapid weathering of granitic rocks in warm, humid climates and can be purified and converted directly into alum.
Potassium alum is soluble in seven times its weight of water at room temperature and is very soluble in hot water. When crystalline potassium alum is heated, some of the water of hydration becomes chemically separated, and the partly dehydrated salt dissolves in this water, so that the alum appears to melt at about 90 degrees C (approx. 392 degrees F), potassium alum swells up, loses all water, and becomes a basic salt called burn alum. Potassium alum has a density of 1.725.
Alum's are used for a variety of uses including as a powerful astringent.
If an aluminum compound, such as aluminum chlorohydrate or aluminum zirconium, which is very soluble, is used as an antiperspirant, that compound is readily absorbed. Once in the body, the aluminum portion of the molecule ionizes, forming free or radical aluminum (Al+++). This passes freely across cell membranes, and forms a physical plug, that when dissolved is selectively absorbed by the liver, kidney, brain, cartilage and bone marrow. It is this concentration of aluminum that has been the source of concern in the medical community and has prompted the research being done on ALZHEIMER'S DISEASE and Breast Cancer
victims. Potassium alum molecules have a negative ionic charge, making it unable to pass through the cell wall. THEY ARE NOT ABSORBED. This is why our deodorants are safe to use and will not cause high levels of ALUMINUM in your system. ALUM and ALUMINUM are two different substances, with distinct chemical signatures. They possess different chemical properties which create different chemical attributes.
We hope this information helps to dispel some of the misunderstandings about ALUM and puts your mind at ease about the safety of all of Deodorant Stones Of America deodorant products.
Reference source: Microsoft® Encarta® 96 Encyclopedia© 1995”
Product link: http://www.deodorantstones.com/index2.ivnu
made from potassium sulfate and other mineral salts which are crystallized over; period of months, then hand shaped and smoothed contains no aluminum chlorohydrate.
No relationship or interest with any of the products mentioned, just found pertinent information in their sites…