Dissolution Mortar Analysis

We offer visual and dissolution analysis of mortar samples to determine mortar composition and recommend a replacement mix.

Please send a sample of plaster, render, or mortar at least 20g in size, and include all undercoats for any render or plaster samples. Please post it in a padded envelope or wrap it in bubble wrap.

We aim to provide a comprehensive emailed analysis report within 14 working days. Where there are different layers of render or plaster, we will analyse the earliest layers and the principal undercoats. If you require analysis of each layer, then we can do that, but you will need to request this and order the appropriate number of samples to be tested.

Please post to the following postal address: 

Mark Womersley at 

M. Womersleys Ltd

Office 2.18
Holmfield Mill
Holdsworth Road
Halifax
HX3 6SN

More about our dissolution analysis:

The dissolution technique, commonly called acid digestion, is a fundamental wet-chemical method for determining the binder-to-aggregate ratio in historic lime mortars. The process relies on the lime binder's solubility in acid, leaving the aggregate residue intact for quantification and further analysis.

The Dissolution Process

The standard procedure generally follows these sequential steps:

1. Preliminary Examination & Preparation:

Before any chemical treatment, a small sample is examined under a microscope to assess its composition and check for soluble components that might interfere with the analysis. The sample is then dried to a constant weight (often at 40°C ) to remove moisture. It may then be gently crushed.

2. Acid Attack:

A weighed portion of the prepared sample is placed in a beaker. A known volume of dilute hydrochloric acid (HCl) is gradually added. The mixture is often stirred and may be heated. While a room-temperature attack is used, research indicates that a hot attack (e.g., in a boiling water bath) is often more effective for rapidly and completely dissolving the binder fraction. The acid reacts with the calcium carbonate (and any hydraulic components) in the binder, dissolving it and releasing carbon dioxide (CO₂) gas.

3. Filtration and Washing:

Once the reaction is complete, the mixture is immediately filtered to separate the dissolved binder from the solid aggregate residue. The residue is thoroughly washed with distilled water to remove any remaining acid and dissolved salts.

4. Drying and Weighing:

For each of the six different British Standard sieve gradings, the washed, insoluble residue (the aggregate) is dried to a constant weight, weighed and photographed under a microscope. The difference between the initial sample weight and the final residue weight estimates the amount of acid-soluble binder originally present, and the sieve separation provides an aggregate grading. We include photographs of all gradings in our reports.

5. Post-Dissolution Analysis

Based upon over 30 years of carrying out this analysis, we produce an informative and useful report together with a matching mortar specification if required. 

Limitations and Considerations

* **Soluble Aggregates**: The method assumes the aggregate is insoluble in acid. If the mortar contains acid-soluble aggregates (e.g., limestone sand or crushed shell), the results will be inaccurate.
* **Hydraulic Binders**: If the mortar contains hydraulic components (e.g., from pozzolans or hydraulic lime), a portion of these may also dissolve in acid. This can lead to an overestimation of the "lime" binder content, and we apply the necessary corrections.
* **Representative Sampling**: The accuracy of the result depends heavily on the sample being representative of the whole mortar. The preparation (crushing and homogenization) is a critical step that strongly affects the final results.

In summary, acid dissolution is a practical and widely used method for quantifying the core components of a lime mortar, though its results are best interpreted alongside mineralogical analysis (like Thermogravimetric Analysis) to account for potential interferences. We also carry out such analysis, often following on and guided by the dissolution analysis.