Silicon Dioxide (Silica): Uses, Grades and Safety

Silicon dioxide, usually just called silica (सिलिका), is the oxide of silicon. Sand and quartz are natural crystalline forms. Most fine white silica powders sold for industry are amorphous forms, such as precipitated or fumed silica. They're light, fluffy and very fine.

• Formula: SiO2
• CAS number: 7631-86-9 (the number generally used for amorphous silicon dioxide)
• Molar mass: about 60.08 g/mol
• Melting point: about 1,713 °C
• Solubility: practically insoluble in water and in most acids, but attacked by hydrofluoric acid and hot strong alkalis
• Appearance: white, odourless powder

Silica doesn't react with most things you'll mix it into. That's why it's used to change texture, thickness and finish rather than chemistry.

Azlok sells silicon dioxide powder in a 100 g pack, labelled Industrial and Utility Grade.

A 100 g pack of fine silica is quite bulky, because the powder is so light. See today's price per kg for the current rate.

• Resins and adhesives: fine silica thickens epoxy and polyester resin so it doesn't run on vertical surfaces. It's also used to make fillets and gap-filling pastes.
• Paints and coatings: a flatting (matting) agent that reduces gloss, and an anti-settling aid.
• Rubber and plastics: a reinforcing filler, especially in light-coloured rubber goods where carbon black can't be used.
• Ceramics and glass: silica is the main glass-former in glazes and glass batches.
• Polishing: a mild abrasive in some polishing pastes.
• Food and pharma: special grades are used as anti-caking agents, but technical grade is not for these uses.

There's no single dose for silica. How much you need depends on the resin or paint, the particle type and the thickness you want. Work it out on a small batch.

1. Mix the liquid first. For two-part epoxy, combine resin and hardener fully before adding any silica.
2. Add powder in small portions. Stir slowly after each addition, folding rather than whipping, so you don't throw dust into the air or trap bubbles.
3. Check the texture after each portion. Stop when it holds the shape you need, like a soft paste for gap filling or a non-sag gel for vertical work.
4. Weigh what you added against the weight of the liquid, and write it down. Next time you can repeat it exactly.
5. For glazes and glass, silica goes in as a percentage of a tested dry recipe. Don't change it by eye, because the glaze can craze or turn dry and matt.

Silica powder is not the same as silica gel. Don't use this powder as a desiccant sachet.

• Dust is the main concern. Fine silica floats easily. Wear a well-fitting dust mask (N95 or better), safety glasses and gloves, and work with good ventilation.
• Breathing crystalline silica dust over time can cause silicosis, a serious lung disease. Amorphous silica isn't linked to silicosis, but its dust still irritates the lungs. Treat all silica dust with care.
• Pour slowly and close to the container, and never sweep spills dry with a stiff broom. Wipe them up damp instead.
• Keep it away from hydrofluoric acid and hot strong alkalis.
• Store it tightly closed and dry. Silica absorbs moisture and smells from the air.
• Keep it away from children, food and kitchen containers.
• Small amounts can go in normal solid waste, bagged so they don't blow around. Don't wash large amounts down drains.

For resin art, model making, small repairs or a few glaze trials, the 100 g technical pack is the sensible start. It goes further than you'd expect because the powder is so light.

If you need silica for food, cosmetics, medicines or analytical testing, technical grade is the wrong choice. Ask for a grade made for that purpose. Paint, rubber and composite units needing 25 kg or more can share their requirement on our bulk inquiry page. Mention whether you need fumed or precipitated silica, and any surface area or particle size specification.