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Behind the Scenes in Conservation: Interpreting x-rays, conservation-style

by Conservation


behind the scenes , conservation , Zaragoza , retablo of saint peter , X-ray

Our paintings conservator is still poring over the x-ray images taken during a recent demonstration of digital x-ray imaging at the museum courtesy of Cory Morriss from GE and Scott Hunley of RCON NDT.  Here’s a detail of one of the paintings from Zaragoza’s “Retablo of Saint Peter.”  At left is the same detail in normal light, after conservation. 

There is a lot of structural information in the x-ray image.  Most noticeable are the two large knots in the wood made visible by the excessive gesso underlayer that was needed to smooth out the wood panel where they were located.  No doubt the knot on the right contributed to the losses in the paint that we see on the surface. 

The vertical wood grain is visible through the whole x-ray, and a closer look shows the layer of woven fabric that lies under the paint, between the wood and gesso layer.  Large losses to the paint and gilding are also apparent as interruptions in the painted areas. 

The two large spikes entering from the left are remnants of the altarpiece’s original construction.  These are the ends of very long hand-wrought iron nails that were hammered into the large wood altarpiece as it was being constructed, before any painting or gilding was done.  The two bright white spots in the painting are the rectangular heads of nails that were hammered straight through the panel from the front, also before painting.  Like the spikes on the left, these were clipped off (on the back) when the retablo was taken apart.  

If you look closely, you can see a faint empty horizontal hole below the spikes on the left.  This would have held a long wooden peg to line up and attach the panel to its neighboring painting when the finished altarpiece was being assembled onsite in the church.  The peg was removed when the altarpiece was taken apart.

Finally, the perfectly round white dot along the left side is a slotted screw that was inserted to fasten the frame member to the painting.  The screw, though modern, dates to before 1960, when the museum acquired the retablo.

Finding this kind of structural information in every painting in the “Retablo of Saint Peter” can help us reconstruct it and answer questions such as: Are the scenes from the life of Saint Peter organized in original positions? Do we have all the scenes?  If some are missing, how many might there be?  Do all the pieces belong to one altarpiece?  Can we match any details to some other painting that might be a part of the original altarpiece? 

And of course, more practically, having an x-ray image can give the conservator a lot of help in determining the condition of the painting before beginning treatment.