Alum Uses

Alum Uses: A Practical Guide to Applications, Benefits and Safety

Alum is a family of water-soluble double salts, most commonly referring to potassium alum (potassium aluminium sulfate) and ammonium alum (ammonium aluminium sulfate). These compounds are widely used in water treatment, personal care, leather processing, food, and several industrial processes. Modern alum products are manufactured internationally by specialist chemical producers such as Basf, Merck, and GAC Chemical, and supplied into municipal, industrial and consumer markets.

Below is a factual, source‑based guide to the main alum uses, with references to credible technical and industry resources.

1. What Is Alum?

“Alum” typically refers to hydrated double sulfates of aluminium and another monovalent cation (such as potassium or ammonium). Chemically, common alums have the general formula:

MAl(SO₄)₂·12H₂O,
where M is a monovalent cation such as potassium (K⁺), ammonium (NH₄⁺), sodium (Na⁺), or others.

For example:

Potassium alum (potassium aluminium sulfate dodecahydrate) – KAl(SO₄)₂·12H₂O – is described in technical data sheets and product catalogues from manufacturers such as GAC Chemical and Merck as a colourless to white crystalline solid used as a coagulating, astringent and mordant agent in various applications.
Ammonium alum (ammonium aluminium sulfate dodecahydrate) – NH₄Al(SO₄)₂·12H₂O – is similarly documented as a water‑soluble crystal used in water treatment, cosmetics and other industries.

These descriptions are consistent across multiple chemical supplier specifications, including Merck’s product entry for potassium aluminium sulfate dodecahydrate.

2. Alum Uses in Water Treatment

2.1 Coagulant in Drinking Water and Wastewater

One of the most important alum uses is as a coagulant in water and wastewater treatment. When alum is added to water, it hydrolyses to form aluminium hydroxide flocs that bind suspended particles and colloids, allowing them to settle or be filtered out.

The United States Environmental Protection Agency (US EPA) describes aluminium-based coagulants (including aluminium sulfate and related alum salts) as widely used in drinking water treatment for turbidity removal and to reduce natural organic matter, which can form disinfection by‑products when chlorinated. The EPA’s guidance on coagulants notes their role in conventional filtration processes where alum promotes particle aggregation and sedimentation in clarifiers and filters.
Technical bulletins from water-treatment suppliers such as Kemira explain that alum improves removal of suspended solids, colour and some metals from raw water, and is also used in industrial effluent treatment to meet discharge standards.

These uses are documented in the EPA’s drinking water treatment guidance and industry literature on coagulation and flocculation.

2.2 Phosphorus Removal in Wastewater

Alum is also used for phosphorus precipitation in municipal and industrial wastewater treatment:

Guidance from the Water Environment Federation (WEF) and case studies from utilities show that aluminium salts, including alum, react with phosphate to form insoluble aluminium phosphate, which is then removed as sludge. This helps wastewater plants meet nutrient‑discharge limits and reduces eutrophication in receiving water bodies.

3. Alum Uses in Personal Care and Cosmetics

3.1 Deodorants and Antiperspirants

Potassium alum and ammonium alum appear in many “mineral” or “crystal” deodorant products:

The Cosmetics Ingredient Review (CIR) and the European Chemicals Agency (ECHA) list potassium aluminium sulfate and ammonium aluminium sulfate as cosmetic ingredients used primarily as astringents and deodorant agents.
Product descriptions from manufacturers of crystal deodorants state that potassium alum works by forming a thin, invisible layer of salt on the skin that inhibits the growth of odour‑causing bacteria rather than blocking sweat ducts, distinguishing it from many antiperspirants.

The European Commission’s CosIng database, which catalogs cosmetic ingredients, also classifies potassium alum as a cosmetic astringent and deodorant component.

3.2 Styptic and After‑shave Products

Alum blocks and sticks are traditionally used as styptic agents to stop minor bleeding:

Barbering and shaving references, as well as product information sheets from grooming brands, explain that potassium alum causes protein precipitation and vasoconstriction on the skin’s surface, helping to seal small nicks and cuts after shaving.
The CIR safety assessment describes alum’s astringent effect on skin and mucous membranes, which underpins its use in after‑shave blocks and in some haemostatic pencils.

3.3 Astringent and Skin-toning Uses

Because alum contracts tissues and reduces secretions, it appears in some astringent lotions and facial products:

Cosmetic formula references and ingredient handbooks list alum as an astringent that can temporarily tighten skin and reduce oiliness, though they also note potential skin irritation at higher concentrations, supporting the need to stay within regulated use levels set by bodies such as the EU Scientific Committee on Consumer Safety (SCCS) and national cosmetic regulations.

4. Alum Uses in Food and Food Processing

Alum has several niche uses in the food sector, though many jurisdictions strictly regulate or limit its use:

4.1 Food Additive and Firming Agent

The US Food and Drug Administration (FDA) lists aluminium potassium sulfate (potassium alum) as a generally recognized as safe (GRAS) substance for specific technical uses in food, such as a firming agent in pickles and some baked goods, under 21 CFR §184.
Traditional pickling recipes and food industry references note that small amounts of alum can help maintain crispness in cucumbers and other vegetables. The FDA regulation specifies maximum levels and conditions of use.

4.2 Baking and Leavening Systems

Historical baking powder formulations used alum in combination with sodium bicarbonate as the acid component to produce carbon dioxide gas. Modern formulations may use sodium aluminium sulfate or other aluminium salts, but alum-type compounds are still referenced in food chemistry texts as part of double-acting baking powders.

National regulations, such as those enforced by the European Food Safety Authority (EFSA) and the FDA, place limits on aluminium intake from food additives, leading many manufacturers to reformulate to lower‑aluminium or aluminium‑free systems.

5. Alum Uses in Textile and Leather Industries

5.1 Mordant in Dyeing

Alum is a classic mordant in natural dyeing:

Textile chemistry manuals and dyeing guides describe aluminium salts (particularly potassium alum) as mordants that bind dyes to textile fibres like wool, silk and cotton, improving colour fastness and brightness.
The alum is typically used in a pre‑mordanting or simultaneous mordanting bath where it forms coordination complexes with dye molecules and fibre functional groups.

5.2 Leather Tanning and Dressing

Leather industry references note the use of alum tanning (often in combination with salt) for producing light, soft leathers such as glove leather.
This “tawing” process is distinct from chrome tanning and yields leather that can be reversed by prolonged exposure to water, reflecting the differing stability of alum-treated collagen compared with chromium‑tanned hides.

Technical data sheets from leather‑chemicals suppliers confirm the continued use of alum-based recipes in niche applications.

6. Alum Uses in Construction and Engineering

6.1 Setting Regulator in Concrete and Plaster

Alum has niche applications in cement and plaster systems:

Construction materials references mention that aluminium sulfate and related alums can influence setting time and early strength development in certain gypsum and cement formulations.
Some specialised plasters incorporate alum to adjust setting characteristics for decorative or restoration work, although modern admixtures are more commonly based on other chemistries (e.g., calcium sulfate hemihydrate modifiers, organic retarders).

6.2 Waterproofing and Soil Stabilisation (Historically)

Historical civil engineering texts describe the use of alum solutions for waterproofing masonry and for limited soil stabilization, where alum-induced flocculation and salt crystallisation alter permeability. Such uses have largely been superseded by more effective polymer and cementitious systems but are documented as part of alum’s engineering history.

7. Alum Uses in Paper, Printing and Photography

7.1 Sizing Agent in Papermaking

In traditional acid papermaking, aluminium sulfate (often described in older literature as “papermaker’s alum”) was used with rosin sizes to provide internal sizing that controlled paper absorbency and ink hold‑out.
Technical histories from the Technical Association of the Pulp and Paper Industry (TAPPI) and other papermaking references explain that alum precipitated rosin onto cellulose fibres, giving the paper some water resistance.

While modern papermaking has largely shifted to neutral or alkaline sizing systems using synthetic agents and calcium carbonate fillers, alum’s role is well documented in the evolution of paper chemistry.

7.2 Hardening Gelatin in Photography

In traditional silver‑gelatin photographic processes, potassium alum was sometimes used as a hardener for gelatin emulsions and in fixer baths to improve the mechanical strength of the emulsion layer on film or paper.
Photographic chemistry manuals note that alum promotes cross‑linking and reduces swelling of gelatin, improving handling but requiring careful control to avoid brittleness.

8. Alum Uses in Fireproofing and Flame Retardancy

Some flame-retardant formulations for fabrics and wood historically included alum in combination with other salts (e.g., borax, ammonium phosphates).
Fire-safety manuals and older textile treatment references record that these mixtures could reduce flammability by promoting char formation and releasing non‑combustible gases when heated.

Modern fire-retardant systems have largely moved towards more specialised organophosphorus, nitrogen-based, and intumescent chemistries, but alum is still cited in traditional and low‑cost treatments.

9. Alum Uses in Medicine and Pharmacy (Regulated)

Alum’s astringent and protein‑precipitating properties underpin several regulated medical and pharmaceutical uses:

9.1 Topical Astringent and Hemostatic

Pharmacopoeial monographs (e.g., in the United States Pharmacopeia) list alum as a topical astringent and mild hemostatic agent used externally to reduce minor bleeding from abrasions or small cuts.
Some over‑the‑counter haemostatic sticks and powders for minor cuts, nosebleeds or shaving nicks are based on alum or related aluminium salts.

9.2 Adjuvant in Vaccines (Aluminium Salts)

While not the same chemical as potassium alum, aluminium salts—notably aluminium hydroxide and aluminium phosphate—are widely used as vaccine adjuvants:

The World Health Organization (WHO) and US Centers for Disease Control and Prevention (CDC) explain that these aluminium-based adjuvants help enhance the immune response and have been used in human vaccines for decades.
Technical vaccine monographs note that the term “alum” is sometimes colloquially used for these adjuvants, although the actual compounds are usually aluminium hydroxide or aluminium phosphate rather than potassium alum.

10. Alum Uses in Household and Traditional Applications

In addition to industrial and regulated uses, alum appears in various household and traditional practices, some of which are still common in certain regions:

Pickling and preserving: As noted above, small, regulated amounts of alum can be used to maintain crispness in pickles, as recognised by the FDA.
Home water clarification: Public health campaigns in some countries have recommended the cautious use of alum to settle turbid surface water before boiling, by adding a small measured quantity and allowing flocs to settle. WHO-affiliated water‑treatment manuals describe this as an emergency or low‑resource option, emphasising careful dosing and subsequent boiling or disinfection.
Crafts and DIY dyeing: Potassium alum is widely sold to hobbyists for mordanting fibres in natural dyeing and for marbling paper, consistent with its documented role in textile and paper industries.

These uses are supported by technical guidance from food safety authorities, water‑treatment manuals, and traditional craft literature.

11. Safety, Handling and Regulatory Considerations

Because alum is a source of soluble aluminium, its use is governed by safety and regulatory frameworks:

Occupational exposure: Safety data sheets (SDS) from major manufacturers such as Merck and BASF classify alum as an irritant at high concentrations, recommending protection against dust inhalation and contact with eyes or broken skin.
Drinking water: The World Health Organization’s Guidelines for Drinking‑water Quality discuss aluminium in treated water, noting that residual aluminium should be kept as low as reasonably achievable and that optimised coagulation with alum can balance effective treatment with minimal residuals. National guidelines (e.g., US EPA secondary maximum contaminant level) set advisory limits for aluminium in distributed water.
Food and cosmetics: Bodies such as the FDA, EFSA, and the EU’s SCCS set specific migration limits, maximum permitted levels and conditions of use for aluminium-containing additives and cosmetic ingredients. The SCCS has evaluated aluminium exposure from cosmetics and concluded that safety depends on cumulative exposure from multiple sources.

Users of alum—whether in industry, utilities, or consumer products—are expected to comply with these regulatory frameworks and follow the handling guidance in supplier SDS documents.

12. Summary of Key Alum Uses

Across industries, the main alum uses include:

Water and wastewater treatment: Coagulant for turbidity removal and phosphorus precipitation, as documented by the US EPA and WEF.
Personal care and cosmetics: Deodorant, astringent and styptic agent, referenced in the CIR and EU CosIng ingredient databases.
Food and food processing: Firming agent in pickles and component in some leavening systems, under tightly controlled conditions per FDA regulations.
Textile and leather: Mordant in dyeing and component in alum tanning, as described in textile and leather chemistry references.
Paper, printing and photography: Traditional sizing agent in acidic papermaking and gelatin hardener in photographic processes.
Construction and engineering: Historical and niche uses in setting regulation, waterproofing and soil treatment.
Medicine and pharmacy: Topical astringent and haemostatic; related aluminium salts as vaccine adjuvants, documented by WHO and CDC.
Household and traditional: Pickling, home water clarification under guidance, and craft dyeing and marbling.

When used within regulatory limits and with appropriate handling, alum remains a versatile and widely applied chemical in modern industry, utilities and consumer products.