How Polishing Powders Work
By Frederick M. Hueston
Polishing powders are a staple in marble restoration. They can take a stone that looks lifeless and give it a crisp, reflective shine. To most people it just looks like “magic white stuff,” but the real story is a mix of chemistry and abrasion. If you understand both, you will know how to get the results you want and avoid the problems that frustrate less experienced contractors.
The Role of Oxalic Acid
Marble is made mostly of calcium carbonate (CaCO₃). Polishing powders contain oxalic acid (H₂C₂O₄), which reacts with calcium carbonate in the presence of water and friction. The reaction forms calcium oxalate (CaC₂O₄), along with water and carbon dioxide:
Reaction formula:
CaCO₃ + H₂C₂O₄ → CaC₂O₄ + H₂O + CO₂
That thin layer of calcium oxalate on the stone surface is harder and more reflective than the original calcium carbonate. It gives the stone a deeper, more permanent shine than you can get with abrasives alone.
⚠️ Limitation: Oxalic acid only works on calcium-based stones like marble, limestone, and travertine. On granite, quartzite, or manmade quartz, nothing will happen because there is no calcium carbonate to react with.
The Role of Abrasives
Polishing powders also contain fine abrasives, similar to extremely high-grit sandpaper. These abrasives smooth the surface while the oxalic acid is working.
Think of it as:
- Sanding – abrasives reduce scratches.
- Chemically sealing – oxalic acid bonds with marble to form calcium oxalate.
Together, they create the polished look professionals aim for.
Hot vs. Cold Powders
Polishing powders are often categorized as “hot” or “cold”:
- Hot powders – contain more oxalic acid. They react quickly and create shine fast but can cause problems if misused.
- Cold powders – contain less oxalic acid. They work more slowly and gently, making them safer for softer or delicate stones.
Orange Peel and Why It Happens
If polished marble ends up looking uneven, wavy, or dimpled, this is called orange peel, named after the texture of an orange skin.
Causes of orange peel:
- Powder too hot (high acid concentration or overheated surface)
- Too much dwell time with a hot powder
- Excessive pressure or heat buildup from the machine
- Using hot formulas on soft or delicate marble
Once orange peel develops, the only fix is to re-level the surface with abrasives (such as diamond pads) before repolishing—adding time and cost.
Practical Considerations
- Identify the stone first. Polishing powders only work on calcium-based stones.
- Select hot or cold powders based on the stone’s hardness and sensitivity.
- Use the right water balance.
- Too much = diluted chemical reaction
- Too little = drag, friction, uneven polish
- Keep machines moving in overlapping passes to prevent overheating.
- Watch for orange peel and adjust technique or powders immediately if it begins forming.
Key Takeaways for PROS
Polishing powders are not magic—they are a balance of chemistry and abrasion.
- Oxalic acid reacts with calcium carbonate to form a harder, shinier surface.
- Abrasives smooth out scratches.
- Hot powders speed things up but risk orange peel.
- Cold powders give more control and reduce risk.
Understanding how polishing powders work, how orange peel forms, and how to prevent it will save you from costly rework and put you firmly in control of your polishing results.
