What is alum

Alum was found in almost every household until 100 years ago, and it was also used in many trades. Potassium aluminum alum, or potassium alum for short, was particularly versatile, as it does not lead to discoloration in important applications like iron or chrome alum.
Potash alum dissociates completely in aqueous solution to form potassium, aluminum and sulfate ions.

Medicalthe astringent property was used to stop bleeding or to close open, weeping wounds. Here, too, the networking of body protein, coagulation, is in the foreground. This even made it possible to coagulate the blood of people suffering from haemophilia, who lack the natural coagulation factors. The antibacterial effect was also beneficial in wound treatments. Alum was even used internally by reducing the absorption of harmful substances through the intestinal wall. Today, however, we must warn against all of these applications.
A modern form of application until not so long ago was the razor stick (blood stopper), an alum preparation for quickly closing small shaving cuts.
Also cosmeticone used the astringent and bactericidal effect. Alum made an early deodorant and antiperspirant with it. The "contraction" of the skin pores reduced perspiration and at the same time inhibited odor-causing bacteria. Today, large alum crystals are again being marketed as "natural deodorants". Alum baths are also occasionally used on damp hands or feet.
Conservativehas a particular effect on the killing of bacteria and the formation of dense protein layers (egg white). For example, it was common to soak raw eggs in an alum solution.
As Wood preservativesalum was also used in the past. This was used to protect beam constructions of half-timbered buildings from animal pests. In some historical buildings you can still find the roof beams covered with this salt. At the same time, alum also formed a limited flame protection, as its relatively high proportion of crystal water is released when heated and inhibits the ignition of the wood: alum-treated wood becomes moist when heated.
bleachingwas particularly important in the textile industry and papermaking. Here the bleaching effect combined with the second main effect of alum, the pickling effect. However, the aluminum ions are responsible for this.
At the Staining and dyeingof textile fibers and leather, the aluminum ions mediate chemically between natural fibers and additives to be applied, e.g. dyes, through the formation of complex compounds, only in this way can additives be permanently bound. This is shown again in more detail with the paper sizing. Tissues were boiled in a hot alum solution before they were soaked in aqueous extracts from dye plants. Even the Romans dyed fabrics in this way and produced the typical bright red toga. The main coloring plants were madder root (yellow and brown), red dye (red, imitation for the precious snail purple) and blue wood.
Paint manufacturemade from natural plants was another important application of alum. While in textile dyeing the dye was bound to the fabric by means of a pickling effect, a color pigment had to be created first, which could be mixed into a binding agent, to produce paint and opaque colors. Alum was also used for this. In doing so, the process was exactly the opposite of that used in alum factories. In an aqueous solution with potash and caustic soda, alum is converted back into the finest white clay crystals, to which natural pigments bind. A spreadable pigment composition was obtained in this way.
The GluingE.g. paper corresponds chemically to staining and dyeing. The term is a bit misleading and has nothing to do with gluing. Therefore it should be explained briefly:
Until the 19th century, paper was made from rags and occasionally fiber plants and was accordingly still expensive. At that time, alum was only used to bleach and strengthen rags or plant fibers. In 1765, J.C. Schiffer developed the wood pulp process and thus created the basis for the mass production of paper from wood cellulose. However, wood pulp paper is initially unsuitable for writing on or printing, as it allows the ink to run, similar to blotting paper. In order to avoid running, cellulose paper had to be mixed with a water-repellent material. This prevents it from being sucked through. Abietin resin soap, which was obtained from tree resin by vigorously shaking it with caustic soda, is suitable as such a substance - it is also a product of local forests. The paper previously treated with alum solution was impregnated with this resin soap. The aluminum ions of the alum bind the resin soap permanently to the cellulose fibers by forming complexes.
Gluing, staining and dyeing are comparable processes and differ only in the type and purpose of the additive to be attached: either a preservative, a lubricant (to increase suppleness), a dye or a glue. As we know today, alum or aluminum sulphate had considerable disadvantages as a means of making paper. The sulfate ions introduced at the same time as the aluminum ions are the main cause of the so-called book death. They represent the paper acid that disintegrates books and documents in archives.
Finally, two modern alum applications:
Pink-flowered hydrangeas produce blue flowers when alum is added to the potting soil. Christmas trees or other decorations are sprayed with concentrated alum solution, creating a deceptively real hoarfrost effect after the water has evaporated.
Some instructions on coloring with vegetable dyes and pigment production can be found under 'Sources and Links'.

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