What It Is
Alum, or aluminum potassium sulfate, is a crystal which actually has many uses. In the case of shaving, it's used as an antibacterial and astringent, and as a styptic for small razor cuts or nicks.
An alum block must be wet when used, or applied by rubbing over wet skin. It will tighten skin and close small cuts/"weepers", and speed the healing of razor burn. Alum was originally very important because of its antiseptic properties -- in the age before widespread antibiotics, an infected razor cut could be serious trouble. In fact, one of the first patients treated successfully with penicillin was near death from an infected shaving cut [1].
Alum crystals (and therefore brands of alum block) are generally the same, regardless of the name or brand. Crystals can be more or less homogeneous, more or less pure, and easier or harder to crack, but all shaving alum blocks are made from aluminum potassium sulfate.
Alum blocks and styptic pencils
Both an alum crystal and a styptic pencil are composed of a form of alum. Styptic pencils are usually made from anhydrous aluminum sulfate and many also contain titanium dioxide. They are intended for slightly "worse" cuts or nicks. Styptic pencils usually leave a bit of white residue when dry which can be wiped off with a damp washcloth.
How To Use
To use alum, one should either wet the face, wet the alum block, or both; rub the block over the skin a few times and wait a minute. One can then rinse the face and apply balm, aftershave, and/or cologne as usual.
Alternately, one can leave the alum on your skin, without rinsing it off, followed by an aftershave, balm, or moisturizer. The alum residue can interact with other products and "clot" them, making them lumpy and unsightly, but especially for alcohol-based aftershaves, this method is viable.
Alternately, one can leave the alum on your skin, without rinsing it off, followed by an aftershave, balm, or moisturizer. The alum residue can interact with other products and "clot" them, making them lumpy and unsightly, but especially for alcohol-based aftershaves, this method is viable.
Care of Alum Block
Because the alum block is a crystal, and because it is water soluble, the solution of water and alum can re-deposit on the edges of the block, causing sharp or rough edges. Drying the block after use, and/or wrapping the block in an absorbent cloth or tissue, can keep the edges smooth. Because the crystal is very brittle, dropping an alum block even a short distance will result in cracks.
The following B & B thread Alum Blocks: how do you know when to replace it? has good information about alum. Following is a condensed version of the above referenced thread:
Alum doesn't wear out. It just gradually wears away with use. I think a lot of alum blocks get replaced when they are dropped to the floor and shatter.
There are also alum sticks available. These are convenient to use since they have a handle to hold on to and have a travel case.
Some don't find alum to be beneficial to their shave so they don't use it.
In my experience there are two distinct types of alum block.
This is a pic of the more common, and less expensive, variety. This type is made of compressed alum powder. These blocks have a consistent milky white appearance throughout. These blocks disintegrate in a granular sort of fashion, for lack of a better way to put it, because that's what they are. To use an analogy, think "particle board":
......
Osma alum blocks, and a few others, are more expensive but that's because they're one solid piece of alum cut from a larger stone. They have a sort of "wood grain" to them, a sort of marble-esque look and are somewhat translucent. These types of block take on a very smooth, glassy appearance and feel after a few uses. Keeping to my previous analogy, think 2x4:
.....
This is probably a good time to start thinking about a replacement:
I have an Osma block I got when I first started wet shaving and for several months used it daily. I don't use it very often any more but, on occasion, I'll pull it out. For all intents and purposes it still looks brand new. Maybe a little shinier than when I got it. Barring any unfortunate accidents, I don't anticipate needing to replace it during my lifetime.
The OP mentioned a "hollow", or cavity, forming in their block which is a complaint I have read about before with regard to "poured" alum blocks. This is what it looks like when that starts to happen:
The following B & B thread Alum Blocks: how do you know when to replace it? has good information about alum. Following is a condensed version of the above referenced thread:
Alum doesn't wear out. It just gradually wears away with use. I think a lot of alum blocks get replaced when they are dropped to the floor and shatter.
There are also alum sticks available. These are convenient to use since they have a handle to hold on to and have a travel case.
Some don't find alum to be beneficial to their shave so they don't use it.
In my experience there are two distinct types of alum block.
This is a pic of the more common, and less expensive, variety. This type is made of compressed alum powder. These blocks have a consistent milky white appearance throughout. These blocks disintegrate in a granular sort of fashion, for lack of a better way to put it, because that's what they are. To use an analogy, think "particle board":
......
Osma alum blocks, and a few others, are more expensive but that's because they're one solid piece of alum cut from a larger stone. They have a sort of "wood grain" to them, a sort of marble-esque look and are somewhat translucent. These types of block take on a very smooth, glassy appearance and feel after a few uses. Keeping to my previous analogy, think 2x4:
.....
This is probably a good time to start thinking about a replacement:
I have an Osma block I got when I first started wet shaving and for several months used it daily. I don't use it very often any more but, on occasion, I'll pull it out. For all intents and purposes it still looks brand new. Maybe a little shinier than when I got it. Barring any unfortunate accidents, I don't anticipate needing to replace it during my lifetime.
The OP mentioned a "hollow", or cavity, forming in their block which is a complaint I have read about before with regard to "poured" alum blocks. This is what it looks like when that starts to happen:
Other Uses
Alum crystals (either potassium or ammonium alum) are used as deodorants, because of the same antiseptic and astringent action that makes them useful for shaving. Alum is also used in pickling fruits and vegetables.
Old School barbers would use alum (block) rubbed onto a client's damp hair when said client wore a crew cut-type style. As the hair dried, the alum would cause the hairs to stand up so that any missed hairs could be seen more easily. This would be especially useful for more precision clipper cuts such as a flat top or a brush cut. Alum was also applied to the hair by barbers for general "hair dressing." At home, men in the 1950s would use alum in a similar way as an alternative to pomade or wax when wearing such styles as well. (The "alternative to pomade" part is referenced in the wikipedia article on alum.)
Alum has also shown some usefulness for "problem skin" (ie: oily or adult acne). The alum residue helps to absorb excess oil and it creates an environment hostile to the bacterial growth which can cause acne.
Alum, applied to skin, creates a tacky or sticky sensation. Some shavers find it helpful to rub the tips of their fingers on a wet alum block before shaving, enabling them to better stretch the skin while shaving.[2]
You may wish to visit the wikipedia article on alum (linked below) for more information and many more uses.
Old School barbers would use alum (block) rubbed onto a client's damp hair when said client wore a crew cut-type style. As the hair dried, the alum would cause the hairs to stand up so that any missed hairs could be seen more easily. This would be especially useful for more precision clipper cuts such as a flat top or a brush cut. Alum was also applied to the hair by barbers for general "hair dressing." At home, men in the 1950s would use alum in a similar way as an alternative to pomade or wax when wearing such styles as well. (The "alternative to pomade" part is referenced in the wikipedia article on alum.)
Alum has also shown some usefulness for "problem skin" (ie: oily or adult acne). The alum residue helps to absorb excess oil and it creates an environment hostile to the bacterial growth which can cause acne.
Alum, applied to skin, creates a tacky or sticky sensation. Some shavers find it helpful to rub the tips of their fingers on a wet alum block before shaving, enabling them to better stretch the skin while shaving.[2]
You may wish to visit the wikipedia article on alum (linked below) for more information and many more uses.
Safety Concerns
Keep away from children
Like many shaving supplies, keep alum away from children. If you think a child has eaten or swallowed your alum, seek immediate medical assistance for the child.
Not for deep cuts
Neither styptic pencils nor alum blocks should be used on deep cuts or puncture wounds: they are surface skin treatments only. If in doubt, seek medical attention immediately.