Saint John the Baptist is a painted terracotta sculpture truncated below the shoulders. The provenance of the sculpture can be dated back to 1846, when it was purchased by Eugene Piot in Italy.  It went through several changes of hand before landing in the National Gallery of Art with the attribution of Donatello.  This was changed in 1976 to the studio of Benedetto da Maiano, based on the similarity between this sculpture and da Maiano's documented marble statue of the Young Saint John the Baptist, c. 1480, in the Palazzo Vecchio. The attribution was again changed to Benedetto da Maiano himself in 1984. 
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Two other similar polychromed sculptures are known, one at the Isabella Stewart Gardner Museum and one, a painted stucco version, that was located at the Bode Museum until it was destroyed in a  fire at the end of World War II.

The sculpture is open at the bottom and is hollow inside, except for an internal support wall in the neck.  Tool marks inside show evidence of the scooping motion that was used to remove the clay.

The sculpture is in fair condition overall. There are numerous small paint losses in all paint colors.  The red paint has a mottled appearance and the green paint varies in shades. Examination with ultraviolet light shows the flesh tones with a muted yellow fluorescence and dark areas that correspond to areas of loss or overpaint.

Records and photographs show that the sculpture was restored sometime after 1907. It appears that a dark, monochromatic coating was removed from the surface. In addition, the proper right arm appears to have been altered by extending the arm out further from the shoulder and adding a beveled edge. There is no indication of this alteration on the sculpture today.

Another treatment was carried out in 1983. Again, photographs show that a fair amount of overpaint was removed in the red and flesh tones.  In addition, a shiny overall coating was removed. Areas of bare terra-cotta and a white fill in the chest were inpainted. A photograph of the back shows an olive green strip of paint in between two strips of bright green.

Therrmoluminescence dating of Saint John indicated that the sculpture was last fired between 400 and 700 years ago, which is consistent with the proposed date of construction. The terracotta was previously analyzed in 1998 and was found to have low heterogeneity and low calcium. Analysis in 2011 confirmed the results from the earlier analysis qualitatively.

The sculpture was built up with large blocks of clay, which can be seen in the x-radiographs.  Details were created by adding or removing clay with fine tools and fingers or hands.  The interior of the body and head were hollowed out to reduce the weight and thickness before firing. A short buttressing wall of clay was left in the neck. The sculpture was fired in one piece.  Firing cracks are visible
in the x-radiographs and can be seen in  the interior as well.

​The paint was analyzed using X-ray fluorescence, fiber-optic reflectance spectroscopy, cross-section analysis, polarized light microscopy, and scanning electron microscopy with energy dispersive spectroscopy to identify pigments and binders and to understand the history of the sculpture.

The surface of the terracotta was coated overall with a size layer, most likely some type of protein. No ground was found. Most cross-section samples show that two layers of paint were originally applied with no coatings in between. All of the original paints contain lead white and translucent feldspathic and siliceous fillers. The red paint contains red lake and lead white, while the green paint contains lead white, verdigris, and lead tin yellow. The flesh paint contains lead white and vermilion.The brown paint in the hair and remnants of a brown paint in the hair robe contain brown earth pigments. No evidence was found of an original top varnish layer.  

No remnants of the dark monochromatic layer from the 1907 photograph were found. It is possible to estimate the extent of the manipulation of the proper right arm by mapping the presence of red vermilion paint. In addition, the repair or addition of the knot in the green cloak was painted with emerald green, which was first used in the early 19th century. On the back of the sculpture, the bright green paint appears to be painted directly on the terracotta surface with no size and consists of emerald green and iron earth pigments. It is unclear what the exact circumstances were that led to the two different colors of green paint.  Finally, large areas of the red robe were inpainted at some point in the past and contain mostly earth pigments intermixed with thin fluorescent layers, which may account for the shiny surface in the before treatment photographs from 1983.

In addition to the technical study of Saint John the Baptist, a treatment was also performed.  The treatment was concentrated on the face, neck, and chest.  Cleaning tests were performed with a number of different solvents and solutions. A 0.5% solution of triammonium  citrate in deionized water was chosen based on its relative ease in removing dirt and overpaint without excessive contact time or mechanical action. The citrate solution was mostly effective, however some areas of dirt were held more tenaciously than others.  For this reason, tests were done with a 0.5% solution of ethylenediaminetetraacetic acid (EDTA). It successfully removed all areas of dirt and grime from the surface of the flesh paint. 

​There were two campaigns of inpainting on the surface. The more recent inpaint was easily removed with ethanol, revealing paint losses on the face and neck and a large fill in the chest. The older inpaint did not respond to the ethanol, so was softened by applying acetone to the surface on cotton pads, then was removed mechanically where possible. A small loss in the front green section of the robe was filled and unpainted.  The fill in the chest was mechanically reduced and then refilled to create a more even surface. Golden Mineral Spirit Acrylic Conservation Paints (MSA) were used to inpaint select losses on the surface, after applying a barrier layer of B-72 to prevent the paint from penetrating the terracotta and the fill.  Some losses were not inpainted to leave a general sense of age.