Low frequency electron paramagnetic resonance (LFEPR) spectroscopy

The laboratory is developing LFEPR spectroscopy to non-invasively and non-destructively study objects with cultural heritage significance. LFEPR spectroscopy is a form of electron paramagnetic resonance (EPR) spectroscopy. EPR spectroscopy is used to study paramagnetic materials, such as transition metal atoms and stable free radicals, found in many ceramic, marble, paint pigments, and glazes associated with cultural heritage objects. These signals can be used to identify the clay, pigment, and glaze; and determine the firing temperature of a ceramic object.

Unfortunately, conventional high frequency EPR can only examine objects less than ~125 mm3. This size limitation restricts EPR analysis of objects with cultural heritage significance to shards or a sample of a larger object removed in the least invasive and destructive manner possible. LFEPR on the other hand, can examine much larger objects and therefore can be used to study intact large objects non invasively and non destructively. We have two versions of an LFEPR spectrometer. The first utilizes a 30 cm diameter solenoidal sweep magnet and a small 1 cm or 3 mm diameter radio frequency surface coil. The image to the left is of a ~15 cm diameter, intact, Chinese Ming Dynasty bowl in our solenoidal magnet LFEPR spectrometer. [MRL-2017-1]

The second version of the spectrometer is a unilateral design where the sweep magnet and radio frequency (RF) surface coil are placed adjacent to the sample. Pictureed at the right is the unilateral device referred to as our EPR mobile universal surface explorer (MOUSE). The EPR MOUSE fits in your hand like a computer mouse and is placed against the object to be studied. The EPR MOUSE detects the LFEPR signal of a 3 mm diameter region on the surface of the object. Because the MOUSE is a surface explorer, there is no limit to the size of the object that can be studied. It can be used to non-destructively and non-invasively study objects as small as a pottery shard or larger than a wall mural. We have used our LFEPR spectrometers to study a variety of objects highlighted below. [MRL-2017-2]

The g = 4.3 LFEPR signal from isolated iron(III) ions in a rhombic environment in the fired clay of the Ming Dynasty bowl pictured above in the solenoidal magnet. The 1-cm diameter RF surface coil was used to record the LFEPR signal. Signals from the iron in the clay change with composition of the clay and firing temperature, thus providing a way to assess the origin and firing temperature of a ceramic object.

Linseed oil paint swatches of the pigments ultramarine blue, Han blue, Egyptian blue, blue vitriol, terracotta red, rhodochrosite, coal, and charcoal on canvas. The 1-cm diameter RF surface coil probe was used to record the LFEPR signal from the swatches. The spectra were unique and characteristic of the pigments. [MRL-2018-1]

The EPR MOUSE was used to image the spatial distribution of magnetic ink in a US one dollar bill by recording the ferromagnetic resonance signal from the ink. The magnetic field was kept constant and the EPR MOUSE was rastered across the bill. [MRL-2017-2]

The spatial distribution of the ferrimagnetic resonance signal from the barcode letters RIT laser printed on paper. The signal was recorded by fixing the magnetic field at the value for the signal from electrophotographic toner and moving the EPR MOUSE over the back of the paper. [MRL-2017-1]

The EPR MOUSE was used to record the iron signal from a Meisen candlestick. The spectrum reveals the presence of paramagnetic iron(III) as well as some ferro/ferri-magnetism. [MRL-2017-2]

LFEPR spectra of pairwise mixtures of pigments and a spectral library were used to identify the two pigment component in paint swatches using a least-squares algorithm. Pigments were ultramarine, Han blue, Egyptian blue, blue vitriol, rhodochrosite, terracotta red, and charcoal. [MRL-2020-1]

The EPR MOUSE was used to record EPR spectra from the painting Palm Tree on a Beach at Twilight on a 30x30 point grid. The spectra were classified with an unsupervised interior-point algorithm as terracotta red, blue vitriol, rhodochrosite, or Han blue to produce an image of the painting. [MRL-2021-2]

We recently compiled an open access library of the EPR spectra of paramagnetic pigments. This is intended to be a living library You may contribute entries in the library by emailing Prof. Hornak. [MRL-2022-2]

If the EPR MOUSE can be used to identify mixtures of pigments in paint [MRL-2020-1], why can't it identify underpaintings or hidden layers in paintings? It can. The idea is as follows. The signal from the EPR MOUSE is from the green sampled region. The EPR spectrum from this region is the sum of the spectra in the sampled region. The individual spectra can be extracted to yield the components of surfacce and hidden layers. [MRL-2023-1]

Test target images made with the pigment magnetite in polyethylene of an a) print, and b) print with uniform overpainting. EPR images of the underpainting design produced by rastering the MOUSE sensitive region across the c) print and d) print with uniform overpainting. The red oval represents the size and orientation of the sensitive region of the MOUSE. [MRL-2023-1]

The EPR MOUSE utilizes a unilateral electromagnet to scan the magnetic field and record an EPR signal from an adjacent sample. The magnet requires a variable current DC power supply to sweep the magnetic field which limits the portability of the MOUSE. We recently developed a scannable unilateral permanent (SUPER) magnet for use with the MOUSE. The design is based on two identical diametrically magnetized counter rotating NdFeB ring magnets. The current prototype SUPER magnet can scan between 0 and 100 mT. [MRL-2024-1]

We are currently working on the following LFEPR projects.

• Better understand the LFEPR spectrum of iron. Iron is one of the most prevalent paramagnetic ions in cultural heritage objects. We need to better understand the LFEPR signal from diamagnetic, paramagnetic, ferrimagnetic, or ferromagnetic iron as a function of particle diameter and coordination environment.
  
• Improve the magnetic field field homogenedity of the EPR MOUSE and thus improve the SNR, spatial resolution, and spectral resolution.