Double Portrait. Vilhelm and Ida Hammershøi

Chemical symbol for the element aluminium.

In scanning electron microscopy, backscattered electrons (BSE) are high-energy electrons from the primary beam that bounce back after interacting with atoms in a sample. Heavier elements scatter more electrons, so they appear brighter in BSE images. This contrast helps distinguish areas with different elemental compositions and reveals variations in material density.

Basket weave is a variation of the plain weave where two or more warp and weft threads are woven together in groups, instead of one by one.

Bevelling is the angled shaping of an edge. In canvas paintings the inner edges of the stretcher is often bevelled to prevent imprints or deformations in the canvas.

The binding medium is the material that holds together the pigment particles in paint. It can consist of egg yolk, an oil, glue or a synthetic substance like acrylics. In Hammershøi’s time, artists painted mainly with oil paint.

Literature reference: Bramsen, Alfred and Sophus Michaëlis, Vilhelm Hammershøi. Kunstneren og hans Værk, Copenhagen: Gyldendal 1918.

Alfred Bramsen (1851–1932) was a dentist, author, and art collector. He became Vilhelm Hammershøi’s first patron and biographer, and together with the author Sophus Michaëlis, he published in 1918 the book “Vilhelm Hammershøi. The Artist and His Work”. The book includes a catalogue raisonné of Hammershøi’s works titled Descriptive Catalogue, which lists 379 works (377 numbered entries and 2 with “a” numbers), comprising 335 paintings and 44 drawings from the period 1881 to 1915. Bramsen’s catalogue forms the foundation for all later references to Hammershøi’s works. This also applies to the present database, where survey numbers up to 377 refer to Bramsen’s catalogue, while numbers from 378 onward denote works not included in his listing. New works continue to surface – pieces unknown to Bramsen – which further expand the known corpus of Hammershøi’s oeuvre.

Bruun Rasmussen is a Danish auction house founded in 1948 by Arne Bruun Rasmussen in Copenhagen.

Chemical symbol for the element calcium.

Chemical symbol for the element cadmium.

Chemical symbol for the element cobalt.

Chemical symbol for the element chromium.

A cross section refers to a small paint sample, usually taken from the surface of a painting or from the tacking edges. The sample is embedded in a resin and polished to reveal the layers of the painting. A cross section allows to study the layered build-up of a painting in a microscope and to perform further analysis on individual pigment particles.

Cusping refers to wave-shaped deformations along the edges of a canvas painting, typically caused when a canvas is primed while stretched with cords on a temporary stretcher. Cusping can be primary or secundary depending on when the deformations have occured.

Drying cracks is a specific type of cracks that can occur in the surface of a painting if the top paint layer dries faster than the layer beneath, or if too much binding medium has been used. Drying cracks typically emerge shortly after the painting is completed and while the paint dries, whereas aging cracks occur over a longer period of time.

FORS (fiber optic reflectance spectroscopy) is a non-destructive analytical method that measures the reflectance of light across the ultraviolet, visible and near-infrared regions of the spectrum to determine the chemical composition of pigments and some organic compounds at a molecular level. It is especially useful for identifying natural and synthetic dyes, often in combination with other analytical techniques.

FTIR (fourier transform infrared spectroscopy) is an analytical technique commonly used to identify organic compounds based on their characteristic absorption of infrared radiation. It is particularly useful for the analysis of varnishes, binding media and degradation products present within the layers of a painting. It can be performed in reflection mode directly on a painting or applied to micro-samples taken from the artwork under study.

Chemical symbol for the element iron.

The frame serves both protective and aesthetic purposes and can be original to the artwork or added at a later time. Historical frames may provide valuable information about the artwork’s provenance, often through inscriptions, labels, or stamps found on the reverse side.

The ground layer is a preparatory layer applied directly to the support (for instance a canvas or a wooden panel) to create a smooth, absorbent, and stable surface for painting. It is typically opaque and monochrome in colour, providing a neutral base that influences the subsequent application of paint layers and the final appearance of the painting. The composition of the ground layer may vary depending on the type of support and the historical period and geographical context of the artwork.

Chemical symbol for the element mercury.

Literature reference: Hvidt, Annette Rosenvold and Oelsner, Gertrud, Vilhelm Hammershøi. På sporet af det åbne billede, Copenhagen: Lindhardt and Ringhof 2018.

IR-FC stands for infrared false-colour. It is a processing technique that combines channels from two images captured with the modified camera: visible and reflected infrared, resulting in composite false-colour image. By assigning the infrared signal an artificial colour (typically red), it is sometimes possible to distinguish between different pigments and materials that may otherwise appear identical in visible light. The recording is typically made with a camera sensitive to both visible and infrared light, where the different spectral ranges are subsequently combined digitally into a single image.

IR-L-UV stands for ultraviolet-induced infrared luminescence (can be found as UVIL in the literature). It is a technique used for mapping materials by excitation in the ultraviolet region of the spectrum (max 356nm) in order to capture luminescence in the near infrared region (700-1200nm). The technique and theory behind has derived from Visible Induced Infrared Luminiscence (IR-L-VIS) as a complimentary technique in multiband imaging. IR-L-UV can be used to distinguish between anatase and rutile titanium oxide. Anatase pigments generally show no IR luminescence, whereas rutile pigments exhibit IR luminescence.

IR-L-VIS stands for visible induced infrared luminescence (can be found as VIL in the literature). It refers to an imaging technique used for the mapping of certain pigments by illuminating an object with a excitation wavelength in the 400–540nm region, which causes an emission (luminescence) in the near infrared region (NIR) 700–1100nm. A change occurs between the excitation and the emission when the wavelength gets longer and there is a decrease in intensity. This luminescence is most visible with cadmium (red/yellow) pigments and the pigment Egyptian blue.

IR-R-IR refers to infrared reflected photography. A painting is illuminated with infrared radiation, which penetrates the paint layers until it reaches materials that either absorb or reflect the rays within the near infrared spectrum (NIR) of the electromagnetic spectrum (700–1100nm). When a work is illuminated with infrared radiation underdrawings can be made visible if these are carried out with carbon-containing materials on a light-coloured reflective ground layer. Images made in the 850–1100 nm range are within the abilities of modified digital cameras. In this case the CMOS senser is used to capture the image.

IRR refers to infrared reflectography – an imaging technique similar to infrared photography. A painting is illuminated with infrared radiation, which penetrates the paint layers until it reaches materials that either absorb or reflect the rays within a larger range of the near infrared spectrum (700–1700 nm). In this range, many binders and certain pigments become transparent, allowing the underdrawing to appear more clearly if these are carried out with carbon-containing materials on a light-coloured ground layer. Images made in the 900–1700 nm range are within the ability of typical electronic infrared imagers. In this case the InGaAs sensor is used to capture the image.

Registering or aligning images means adjusting two or more images so that the same features or areas match up in the same position. In practice, this involves applying geometric transformations to one or more images to align them with a fixed reference image so they fit precisely on top of another. This allows direct comparison between images taken with different imaging systems, under different conditions, or showing different kinds of information. Precise alignment of images is essential to allow meaningful visual comparison and analysis in an image viewer. However, image registration, the process of spatially aligning two or more images from different systems, can pose some challenges. Each system, such as those used for MA-XRF scanning, infrared reflectography (IRR), and multispectral imaging (MSI), produces data with different resolutions and pixel sizes. Variations in optical distortions, illumination conditions, and sensor characteristics lead to geometric and radiometric inconsistencies. The obtained images may not perfectly overlap, requiring complex geometric transformations to achieve proper alignment. Furthermore, differences in image contrast can make it difficult to identify common features and reference points. For this reason, some images may appear slightly distorted, especially along the edges, when presented in the viewer.

Thickly applied paint.

Monochrome semi-transparent thin paint applied on top of the ground layer to achieve the right colour for the further build-up of the paint layer. The layer also functions as an isolating layer to avoid the absortion of the oil from the paint layer.

In infrared photography (IR-R-IR) a painting is illuminated with infrared radiation, which penetrates the paint layers until it reaches materials that either absorb or reflect the rays within the near infrared spectrum (NIR) of the electromagnetic spectrum (700-1100nm). When a work is illuminated with infrared radiation underdrawings can be made visible if these are carried out with carbon-containing materials on a light-coloured reflective ground layer.

Infrared reflectography (IRR) is an imaging technique similar to infrared photography (see IR-R-IR). A painting is illuminated with infrared radiation, which penetrates the paint layers until it reaches materials that either absorb or reflect the rays within a larger range of the near infrared spectrum (700–1700 nm). In this range, many binders and certain pigments become transparent, allowing the underdrawing to appear more clearly if these are carried out with carbon-containing materials on a light-coloured ground layer.

Kleis’ Kunsthandel in Copenhagen, established 1831, was a gilding and framing firm, as well as an art gallery. The owner in Hammershøi’s time was Valdemar Kleis, whose label is found at the back of several of Hammershøi’s paintings, mainly on the frame. But the shop also offered stretched canvases in a wide range of sizes. After the death of Valdemar Kleis in 1918 the firm was continued by his son Georg Kleis.

Kultorvets Farvehandel was founded in 1879 by F. E. Kühle and was located on the square “Kultorvet” in Copenhagen. The paint shop counted several well-known painters among its customers, and its stamp is also found on the reverse of some of Hammershøi’s works. The shop sold all kinds of artists’ materials as well as tools such as easels, palettes, and paint boxes. Among the products offered were also painting millboards and stretched canvases in a wide range of standard sizes. However, they also offered to supply canvases and stretcher frames in custom formats, which suited Hammershøi’s practice well, as his works rarely adhered to predetermined dimensions. The owner during Hammershøi’s time, Hermann Averhoff, also ran a factory producing tube paints, and it is likely that Hammershøi purchased paints from the shop as well.

Lining is the process of attaching a new canvas to the back of a canvas painting. The purpose of the lining canvas is to reinforce a weakened or damaged original support. Historically, materials such as glue-paste and wax-resin, were commonly used for lining. Today, conservators prefer modern synthetic materials that are more stable and less sensitive to fluctuations in humidity and temperature. Linings were once a standard practice, whereas today conservators emphasise less invasive treatments.

Strip lining refers to a method in which the edges of a canvas painting are reinforced without the lining covering the entire back of the painting.

Loose lining is a conservation technique used to support and protect the original canvas without permanently attaching a new fabric to the reverse.

MA-XRF (macro X-ray fluorescence mapping) is a non-destructive analytical method that scans areas of a painting to produce maps showing the distribution of chemical elements. This method enables the visualisation of pigment compositions across different areas of a painting and can reveal hidden layers, as well as alterations made by the artist or during past conservation treatments. In EDXS and XRF analysis, K, L and M refer to electron shells in an atom. When an electron from a higher shell falls to fill a vacancy in a lower shell, it releases energy as X-rays, which are unique to each element. The spectral line is named after the shell (K, L or M) that receives the electron. By measuring these X-ray emissions, we can identify the elements in a sample or object.

A mitre joint is a type of joint made by cutting the ends of two pieces of material at an angle – usually 45° – so that they fit together to form a corner, typically 90°.

Chemical symbol for the element manganese.

A mortise and tenon is a woodworking joint used to connect two pieces of wood, typically at a right angle. The mortise is a rectangular hole cut into one piece, while the tenon is a  section on the end of the other piece, shaped to fit into the mortise.

Multispectral imaging (MSI) is a technique that captures images of an artwork at different wavelength bands across the electromagnetic spectrum – such as ultraviolet, visible, and infrared light. Each band can reveal specific features that are invisible to the naked eye, including underdrawings, changes to the composition, surface damage, or past restorations.

Chemical symbol for the element nickel.

A technique that uses visible light and lenses to magnify and examine the surface and structure of a painting. When applied to cross sections of paint samples, it allows for detailed observation of the painting’s stratigraphy (layer structure) and pigment particles. It is often employed with various illumination techniques, such as dark field and UV fluorescence, to enhance the analysis.

Paint layers are applied over the ground layer and are composed of pigments or colourants mixed with a binding medium. Throughout history, artists have used various binders: in the Middle Ages, egg yolk was commonly used in tempera painting; during the Renaissance, oil became the preferred medium. In modern times, synthetic binders such as those found in acrylic paints are also widely used. The paint layer forms the visible image of the artwork and is often built up in multiple layers to create effects of colour, texture, depth, and transparency.

Chemical symbol for the element lead.

A plain weave interlaces warp and weft threads one over one.

The provenance of an artwork is the documented history of its origin and chronology of ownership. Knowing the provenance of the artwork is an important factor in the assessment of its authenticity.

In raking light (VISr-R-VIS) the painting is illuminated with 30° visible light from the left-hand side, which emphasises the surface structure of the work and can elucidate changes in the composition and other phenomena related to the making of the work. The colour reproduction in this type of image is not representative, as the work is underexposed in one side.

UV-R-UV refers to reflected ultraviolet fluorescence. With this technique the painting is illuminated with reflected UV radiation (300-400nm). The technique can show surface phenomena such as varnishes and other surface materials.

SEM-EDXS (scanning electron microscopy combined with energy dispersive X-ray spectroscopy) is a technique that provides highly detailed images at the microscopic level while simultaneously identifying the elemental composition of a sample. It is particularly valuable for studying the stratigraphy of paint cross sections at very fine scales, for the chemical characterization of pigments, fillers and degradation products, and for detecting trace elements that may indicate very specific materials. In EDXS and XRF analysis, K, L and M refer to electron shells in an atom. When an electron from a higher shell falls to fill a vacancy in a lower shell, it releases energy as X-rays, which are unique to each element. The spectral line is named after the shell (K, L or M) that receives the electron. By measuring these X-ray emissions, we can identify the elements in a sample or object. The Pb L spectral line in SEM-EDXS is automatically labelled as “Pb” by the EDXS software, while the Pb M line must be added manually. When added, it is labelled “F1”, so elemental maps for Pb M appear with this generic label, though the caption clarifies that they represent the Pb M line.

Scumbling is a painting technique where a layer of paint is applied over a dried layer of another colour, allowing parts of the underlayer to show through. The top layer is applied with a dry brush using a broken motion rather than smooth strokes. It modifies or utilises the colour beneath without completely covering it and creates a shimmering effect.

The selvage is the woven, reinforced edge along the sides of a piece of canvas that prevents the fabric from fraying.

Chemical symbol for the element silicon.

Sizing refers to the process of applying a layer of glue to the support before applying the ground layer. The sizing reduces the absorbency of the support.

pXRF (spot X-ray fluorescence spectroscopy) is a point-based, non-destructive analytical technique that identifies the elements present in a small area on the surface of a painting. It is commonly used to determine the chemical composition of specific locations on a painted surface.

Standard formats, a system dating back to the mid-eighteenth century, were specific formats of ready-stretched canvases sold by artist’s supply shops. The three basic categories Portrait, Landscape and Marine each had their own range of fixed standard formats, numbered according to the individual sizes.

A non-expandable wooden frame on which the canvas is stretched and fixed.

A stretcher is a wooden frame used to stretch and secure a canvas. It is typically designed with expandable joints and small wooden wedges (called keys) that allow adjustments to maintain the tension of the canvas over time. This helps prevent sagging as the canvas responds to aging or changes in humidity. In contrast, a strainer is a similar wooden frame but non-expandable, meaning it cannot be adjusted once the canvas is mounted.

To stretch the canvas on to a stretcher or strainer.

The support of a painting refers to the material on which the ground layer and paint layers are applied. Over time, artists have used a variety of materials as supports, including canvas, wooden panels, copper plates, cardboard, and paper. The choice of support influences the painting’s texture, durability, and how it ages. It can also offer valuable insights into the artwork’s origin, technique, and historical context.

A final transparent layer – typically a varnish of either natural or synthetic origin – sometimes applied as a thin surface coating over the dried paint layer to protect the artwork from dust, dirt, and mechanical damage. In addition to providing protection, varnish saturates the colours and evens out the surface gloss. Over time, this layer may yellow, or degrade. In the 17th century, it was standard practice to varnish oil paintings. In Hammershøi’s time, however, oil paintings were not always varnished, and we know that Hammershøi sometimes deliberately chose to leave his works unvarnished.

The survey numbers are referring to Alfred Bramsens catalogue from 1918. This catalogue includes 377 art works by Hammershøi, whereas paintings not included in the Bramsen catalogue have been given new ViHDA survey numbers starting from 378 onward. New works continue to surface – pieces unknown to Bramsen – which further expand the known corpus of Hammershøi’s oeuvre.

The tacking edge is the part of the canvas that is folded around the edge of the stretcher and used to mount the canvas. Often, both ground layer and paint layers can be seen on the tacking edges.

In tangential light (VISt-R-VIS) the painting is illuminated with visible light in a very shallow angle (5-10°) from the left side, which emphasizes the surface structure of the work and elucidate changes in the composition and other phenomena related to the making of the work. The colour reproduction in this type of image is not representative, as the work is underexposed in one side.

Thread-counting is performed on X-radiographs of canvas paintings to create weave-maps. These are used for analysing the structure of the canvas and to compare canvases used in different paintings. A comparison between weave-maps can sometimes determine if two or more pieces of canvas derive from the same roll and thereby shed light on the place and period in which a painting has been created.

Thread counting is performed by examining high-resolution X-ray images, where the individual warp and weft threads appear as light and dark lines. By counting the number of threads per centimeter in both directions, the canvas density and weave pattern can be visualised in a weave-map. The thread-counting and weave-maps are performed by special software.

In some instances, achieving an accurate thread count may prove difficult if the ground layer, for example, does not sufficiently absorb the X-rays.

In the 19th and 20th centuries, dealers of artists’ materials sometimes stamped or printed their logos or names on the reverse of canvases and stretchers. Trademarks can sometimes help conservators and art historians identify the dealers, as well as the place and period in which the materials were produced and sold.

A twill weave forms diagonal lines by interlacing warp and weft threads in a regular stepped pattern.

UV-A refers to near-ultraviolet light with wavelengths approximately between 320 and 400 nm. In optical microscopy, UV-A light is commonly used as an excitation source to induce fluorescence in a sample.

UV-FC stands for ultraviolet false colour. It is an imaging technique in which a photograph taken under ultraviolet illumination is combined with a regular colour photograph to create a composite image. By assigning the ultraviolet-reflected signals artificial colours, it becomes possible to reveal differences in pigments and materials that are not visible in normal light. The image is typically made with a camera sensitive to UV radiation and relevant filters, after which the different spectral ranges are digitally combined.

UV-I3 refers to a specific fluorescence filter set, such as those used in Leica microscopes. This set typically uses near-UV excitation (around 320–400 nm) and includes a filter that blocks the reflected UV radiation while allowing visible fluorescence (usually above 425 nm) to pass through for observation.

UV-R-UV refers to reflected ultraviolet fluorescence. With this technique the painting is illuminated with reflected UV radiation (300-400nm). The technique can show surface phenomena such as varnishes and other surface materials.

The underdrawing is a preliminary sketch applied directly onto the ground layer, serving as an outline for the composition or parts of it before the paint layers are added. These drawings are often not visible to the naked eye but can be revealed through infrared imaging (IRR and IR-R-IR). The underdrawings can offer valuable insight into the artist’s creative process and planning, showing how the composition evolved prior to the final painting.

The underpainting is an initial layer of paint applied between the underdrawing and the final paint layers, serving as a foundation for the subsequent application of colour. It is often executed in a monochrome palette as a starting point for the final modelling of the composition.

VIS-L-UV stands for ultraviolet-induced visible luminescence. It is a technique in which a painting is illuminated with ultraviolet light (365 nm), causing certain materials on the surface to emit fluorescent light in the visible spectrum (400-700nm). In contrast areas that do not react on ultraviolet radiation will absorb it and appear dark. In this way the technique can reveal varnishes, retouchings, adhesives and overpaints that are not always visible under normal light. VIS-L-UV is particularly suitable for seeing overpaintings, which will often appear as dark areas when they are a certain age.

VIS-R-VIS means reflected visible light and is the same as ordinary photography. The image is recorded under even uniform light (30 degrees from each side). Here, the reflected light is recorded in the visible part of the electromagnetic spectrum when the painting is illuminated with visible light. The image shows what is seen with the naked eye. The recordings made under symmetrical, visible light setting are intended to document the physical properties of the painting precisely at the time of recording rather than an aesthetic representation of the painting.

VISr-R-VIS refers to raking light. The painting is illuminated with 30° visible light from the left handside, which emphasizes the surface structure of the work and can elucidate changes in the composition and other phenomena related to the making of the work. The color reproduction in this type of image is not representative, as the work is underexposed in one side.

VISt-R-VIS refers to tangential light. The painting is illuminated with visible light in a very shallow angle (5-10°) from the left side, which emphasizes the surface structure of the work and can elucidate changes in the composition and other phenomena related to the making of the work. The color reproduction in this type of image is not representative, as the work is underexposed in the right side.

Literature reference: Vad, Poul, Hammershøi. Værk og liv, København: Gyldendal 1988.

A varnish is sometimes applied as a final transparent layer over the dried paint layer to protect the artwork from dust, dirt, and mechanical damage. In addition to providing protection, varnish saturates the colours and evens out the surface gloss. Over time, this layer may yellow, or degrade. Until the 20th century, it was common practice to varnish oil paintings. In Hammershøi’s time, however, oil paintings were not always varnished, and we know that Hammershøi sometimes deliberately chose to leave his works unvarnished.

Thread-counting is performed on X-radiographs of canvas paintings to create weave-maps. These are used for analysing the structure of the canvas and to compare canvases used in different paintings. A comparison between weave-maps can sometimes determine if two or more pieces of canvas derive from the same roll and thereby shed light on the place and period in which a painting has been created.

Thread counting is performed by examining high-resolution X-ray images, where the individual warp and weft threads appear as light and dark lines. By counting the number of threads per centimeter in both directions, the canvas density and weave pattern can be visualised in a weave-map. The thread-counting and weave-maps are performed by special software.

In some instances, achieving an accurate thread count may prove difficult if the ground layer, for example, does not sufficiently absorb the X-rays.

X-radiography is an imaging technique that uses X-radiation to penetrate through the paint layers and the support. Heavy metal-containing materials like lead-based pigments absorb more X-rays and appear lighter on the image, while less dense areas appear darker. This allows hidden features such as changes in composition (pentimenti), underlying sketches, damages, or restorations to be revealed. The X-ray is made by directing x-rays through the painting and capturing the transmitted radiation on a flat-field detector or image plate placed behind it. The image is based on the radio-opacity of the materials (density and thickness). X-ray images of canvas paintings can be used for creating thread counting.

X-radiography is an imaging technique that uses X-radiation to penetrate through the paint layers and the support. Heavy metal-containing materials like lead-based pigments absorb more X-rays and appear lighter on the image, while less dense areas appear darker. This allows hidden features such as changes in composition (pentimenti), underlying sketches, damages, or restorations to be revealed. The X-ray is made by directing X-rays through the painting and capturing the transmitted radiation on a flat-field detector or image plate placed behind it. The image is based on the radio-opacity of the materials (density and thickness). X-ray images of canvas paintings can be used for creating thread counting.

Chemical symbol for the element zinc.