Historic Paint Analysis

Prime Testing Group Ltd. offers the testing services of Historic Paint Analysis:

Paint analysis is about determining first, the number of coatings (including primer and finish coats), the original colors used in a building (often recorded in notations of the Munsell Colors System, a standardized color identification system used commonly in the paint industry), and the types of coatings (i.e. oil or water-based paints, stains, glazes, varnishes, or wallpapers). Analysis can also reveal which colors and paint types are used where in the building, the presence of decorative painting (i.e. graining, marbleizing, stenciling), the physical characteristics (i.e. gloss, texture), and the appropriate date or period of each layer.

Under a laboratory microscope, the analyst records all the layers and matches them to a system of color notation (usually the Munsell Color or the Plochere Color Systems). Through microscopic examination, the analyst can also identify special finishes, such as graining and marbleizing, and can differentiate between primers and finish coats.

A stereomicroscope enables the analyst to count the number of coating layers seen in cross-section, and is also useful to measure relative paint thickness and for observing accumulations of grime and soot between layers of finish paints. A stereomicroscope is especially well suited for finding sample areas that are the least discolored and, therefore, best for color matching.

Another instrument used by the paint analyst is the polarized light microscope. It is used to analyze and identify the pigments used in a paint film, in micro-chemical media tests, or to analyze wallpaper fibers found during investigations. The polarized light microscope is also used to photograph pigment particles or crystals found in a preparation. The cross-sections of paint are often embedded in a "tablet" of acrylic resin. When wet and covered with a cover slip, the cross-section is ready for photography using the top light. Polarized light microscopy (PLM) techniques allow for the identification of different pigment particles based on the characteristics of particle shape, color, refractive index, and optical properties. If PLM requires further confirmation, energy-dispersive x-ray analysis (EDX) may be carried out using a scanning electron microscope.

Ultraviolet light microscopy is also used, especially to identify media. Illumination of different materials with ultraviolet light causes them to auto-fluoresce, or glow, with characteristic colors. For example, shellac will fluoresce orange or yellow-orange, and some plant resins (amber, copal, sandarac, and mastic) fluoresce bright white. Exposing the paint layer to long-wave ultraviolet light can usually reduce the discoloration of oil media. This effect is called “bleaching”. The length of exposure time required varies with different paints, and can be difficult to measure. Often a portion of the sample is covered with an opaque material, which is moved slightly every 24 to 48 hours, until the coloration differences between the sections become negligible.

Chemical tests can be done to determine the actual pigment, or pigments, used as well as to determine paint media. For example, the presence of lead in paint samples is tested for by applying a tiny amount of diluted sodium sulfide reagent through a micropipette onto a specific paint layer or onto all layers in a cross-section. Any black sulfide discoloration reactions that occur clearly indicate which layers contain lead and which do not.

All of the information is then organized in a paint seriation chart, which describes the chromo-chronology, literally the color history of the building. Finally, the paint analyst will recommend the type of modern paint, which may be used to create the correct surface and texture of the original or, in special cases; it may be desirable to mix the paints according to historic recipes, or to increase the paint’s texture through the addition of sand.

EN 17543:2021

Historic Paint Analysis is a tool in the conservation of built heritage. EN 17543:2021 provides a standardized framework for investigating and documenting finishes on historic structures. Historic Paint Analysis under EN 17543 is a multidisciplinary endeavor that blends art history, chemistry, and conservation science. By combining microscopic, chemical, and instrumental methods, conservators can uncover the hidden stories embedded in architectural finishes and guide respectful restoration.

EN 17543:2021, titled Conservation of Cultural Heritage – Finishes of Built Heritage – Investigation and Documentation, outlines best practices for examining decorative and protective finishes on historic buildings. It emphasizes:

• Systematic documentation of findings

• Multidisciplinary collaboration among conservators, scientists, and historians

• Non-destructive and minimally invasive techniques

• Scientific rigor in identifying materials and stratigraphy

The standard applies to both interior and exterior finishes and aims to preserve authenticity while guiding restoration efforts.

Microscopic Examination

Microscopic analysis is the cornerstone of historic paint investigation. It reveals the stratigraphy - the sequence of paint layers - and helps identify pigments and binders.

Techniques:

• Optical Microscopy: Used to examine polished cross-sections of paint samples under visible and UV light. It reveals layer thickness, color, and deterioration.

• Polarising Light Microscopy (PLM): Enhances pigment identification by analyzing birefringence and crystal morphology.

Micro-Chemical Testing

Micro-chemical tests are qualitative methods used to identify specific compounds in paint samples.

Common Tests:

• Acid tests for carbonates

• Solubility tests for binders (e.g., oil, protein, gum)

• Spot tests for metallic pigments (e.g., lead, copper)

These tests are often used in conjunction with microscopy to confirm hypotheses about material composition.

Instrumental Analysis of Pigments and Binders

Advanced instrumental techniques provide quantitative and molecular-level data.

1. Fourier Transform Infrared Spectroscopy (FTIR)

• Identifies organic binders (e.g., oils, proteins, resins) and some inorganic pigments.

• Works by measuring infrared absorption spectra of molecular bonds.

• Non-destructive and suitable for micro4-samples.

2. Gas Chromatography–Mass Spectrometry (GC-MS)

• Separates and identifies volatile organic compounds in binders.

• Ideal for detecting drying oils, waxes, and resins.

• Requires sample derivatization and is more invasive than FTIR.

3. X-Ray Fluorescence (XRF)

• Detects elemental composition of pigments (e.g., lead, zinc, iron).

• Portable XRF devices allow in-situ analysis.

• Non-destructive and useful for mapping pigment distribution.

4. Scanning Electron Microscopy with Energy Dispersive X-Ray (SEM-EDX)

• Provides high-resolution imaging of paint layers.

• EDX detects elemental composition at micro-scale.

• Useful for identifying corrosion products and pigment particles.

Integrating Methods for Comprehensive Analysis

A robust paint analysis combines multiple techniques:

Method Purpose Sample Type Destructiveness

Optical Microscopy Layer structure & color Cross-section Low

PLM Pigment ID via crystal properties Powdered pigment Low

FTIR Organic binder ID Micro-sample Low

GC-MS Binder & additive ID Micro-sample Moderate

XRF Elemental pigment ID Surface or sample None

SEM-EDX Microstructure & elemental ID Cross-section Moderate

Further Information on Historic Paint Analysis

Historic paint analysis, also known as architectural paint research, is a specialized field that combines science, art history, and conservation to uncover the decorative past of buildings and artifacts. By examining layers of paint and finishes applied over time, conservators and researchers can reconstruct the aesthetic and cultural narratives of historic structures.

What Is Historic Paint Analysis?

Historic paint analysis is the scientific examination of architectural finishes - such as paints, varnishes, stains, and decorative coatings - applied to buildings and objects over time. The primary goal is to determine the original appearance of a structure or object and understand how it evolved through successive layers of decoration.

This analysis is not limited to paint alone. It often includes other surface treatments like:

• Varnishes

• Shellacs

• Metallic leaf (gold, silver, bronze)

• Glazes and stains

By identifying the composition, color, and application techniques of these finishes, researchers can:

• Reconstruct historical color schemes

• Date specific layers or renovations

• Understand the cultural and social context of decorative choices

• Guide accurate restoration and conservation efforts

Scientific Foundations and Techniques

Historic paint analysis employs a multidisciplinary approach, combining microscopy, chemistry, and archival research. Here are the key steps and techniques involved:

1. Sampling

• Small samples (often less than 1 mm) are carefully extracted from inconspicuous areas.

• These samples are typically taken from various architectural elements such as woodwork, plaster, metal, or masonry.

2. Sample Preparation

• Samples are embedded in resin and polished to reveal a cross-section of paint layers.

• This cross-section is then examined under a microscope.

3. Microscopic Examination

• Visible light microscopy reveals the sequence of paint layers and their colors.

• Ultraviolet (UV) light can highlight organic materials like varnishes or glazes.

• Polarized light microscopy (PLM) helps identify pigments and fillers.

4. Chemical Analysis

Advanced techniques are used to identify the chemical composition of pigments and binders:

• Fourier-transform infrared spectroscopy (FTIR): Identifies organic compounds like oils and resins.

• Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS): Analyzes elemental composition.

• Gas chromatography-mass spectrometry (GC-MS): Detects complex organic molecules in binders and varnishes.

5. Color Matching and Documentation

• Original colors are matched using standardized color systems (e.g., Munsell, NCS).

• Findings are documented in detailed reports, often including photographs, diagrams, and historical context.

Applications in Architectural Conservation

Historic paint analysis plays a vital role in the restoration and preservation of historic buildings. Some common applications include:

• Restoring original color schemes in historic homes, churches, and public buildings.

• Dating architectural features by correlating paint layers with known historical events or renovations.

• Understanding stylistic changes in interior decoration over time.

• Informing conservation strategies by identifying hazardous materials like lead-based paints.