What Causes Orange Peel on Marble Surfaces

Frederick M. Hueston  StoneForensics.com

Orange peel on marble surfaces can be attributed to several factors, with the excessive use of acid polishing compounds being a significant contributor. Acidic compounds, such as oxalic acid, are commonly used in the restoration process to  polish marble surfaces. However, when used excessively or improperly, these compounds can cause an uneven texture, resulting in an orange peel appearance.

The reaction of oxalic acid with marble can contribute to the formation of orange peel on the surface. Oxalic acid is a commonly used acidic compound in the restoration of marble surfaces, primarily employed to place the final shine on marble surfaces. However, if used improperly or in excessive amounts, it can have adverse effects on the marble, resulting in an uneven texture and the appearance of orange peel.

When oxalic acid comes into contact with the calcium carbonate present in marble, a chemical reaction occurs. Calcium carbonate is the main mineral component of marble and is highly susceptible to the effects of acidic substances like oxalic acid. The reaction between oxalic acid and calcium carbonate can be summarized as follows:

Dissolution of Calcium Carbonate: Oxalic acid, a strong acid, demonstrates its reactivity when it comes into contact with water. Upon dissolving in water, oxalic acid undergoes dissociation, splitting into hydrogen ions (H+) and oxalate ions (C2O42-). The dissociation reaction can be represented as follows:

H2C2O4 ⇌ 2H+ + C2O42-

The hydrogen ions released during this process are responsible for the acidity of the solution. These hydrogen ions then initiate a chemical reaction with the calcium carbonate (CaCO3) present in the marble surface.

The calcium carbonate molecules in the marble consist of calcium ions (Ca2+) and carbonate ions (CO32-). When the hydrogen ions from the dissociated oxalic acid come into contact with the calcium carbonate, they replace the calcium ions, resulting in a reaction known as acid-calcium carbonate dissolution.

The reaction can be summarized as follows:

2H+ + CaCO3 → Ca2+ + CO2 + H2O

In this reaction, the hydrogen ions displace the calcium ions from the calcium carbonate compound, forming water (H2O) and releasing carbon dioxide gas (CO2) as byproducts. As a result, the calcium carbonate dissolves, breaking down into its individual components.

Removal of Surface Material: As the calcium carbonate dissolves under the influence of the hydrogen ions, a thin layer of the marble surface is removed. This removal of material occurs gradually and uniformly, resulting in an uneven texture and surface irregularities. The dissolved calcium carbonate contributes to the formation of a rough and bumpy appearance commonly referred to as orange peel.

The extent of material removal and the resulting texture depend on various factors, including the concentration and duration of exposure to the acid, as well as the characteristics of the marble itself. Overexposure to oxalic acid or the use of high concentrations can lead to excessive removal of the surface material, exacerbating the orange peel effect and potentially damaging the marble.

It is important to note that the orange peel appearance caused by the removal of surface material is generally undesirable in marble surfaces, as it disrupts the smooth and polished finish that is characteristic of this stone. Therefore, it is crucial to use oxalic acid or any other acidic compounds judiciously and with proper care to prevent excessive dissolution and the resulting uneven texture. Thorough rinsing and neutralization of the acid after treatment are essential to minimize the orange peel effect and preserve the integrity of the marble surface.