skip to primary navigation skip to content


Plaster, poultices and Crayola crayons « The Polar Museum: news blog

The Polar Museum: news blog

Plaster, poultices and Crayola crayons

Recently, I began treatment on a sculpted plaster bust by the Danish artist Eigil Knuth. Knuth was born in 1903 and spent many years of his life exploring Greenland, where he conducted archaeological excavations and scientific expeditions. From 1922 – 1924 he studied art at the Copenhagen Academy of Fine Arts, and he spent a further three years studying sculpture in Italy. In 1936 he crossed the Greenland ice cap and then spent the winter on the east coast at Angmassalik, where he made a series of portrait busts of Inuit people he met there. The Polar Museum has two of these busts in its collection; one of a child, and the other of a young woman:


In a treatment in 2014, the museum’s conservator Sophie Rowe did surface cleaning of the sculpture and removed some of the disfiguring dark specks on the plaster. She noted that the yellowish substance around the base looked like what was intended to be a protective layer of wax, but that the object had been left sitting in a bath of it for a long time and it had become a thick, disfiguring coating. Additionally, the hair style of the woman is constructed from two pieces of plaster that were previously attached together, but the joint has broken. The joint on the bun is also covered in this thick, yellow wax which makes it impossible to reattach the pieces with an adhesive:


Plaster can be a tricky material to treat because it is very porous, so it soaks up anything that is applied to it. Even if you are just cleaning with water, the water will soak in and set any dirt or stains even further into the plaster. For this reason, when cleaning plaster, conservators often like to use a technique called poulticing. Poulticing involves putting a solvent (like water, or alcohol) in a substrate like cotton wool, cellulose pulp, or a gel, and applying that to the surface. The solvent in the cotton wool will travel into the plaster, dissolve whatever is causing the stain, and then as it evaporates, carry the stain back out to the surface where the cotton wool is. Thus, instead of just moving around in the plaster, the stain is moved into the poultice, which can then be removed.  Here is a poultice on the hair piece:


Before I could use this technique on the object, I needed to find a solvent that would dissolve the wax but not the plaster, and which could be safely used in our lab. Luckily the wax was very thick on the base of the bust, and it was easy to take small samples of it.  I would have liked to run the samples through an FTIR, which is an analytical instrument particularly good for the identification of organic substances such as waxes, but I wouldn’t have access to one unless we waited several months. Instead I made an educated guess about the identity from a strong-smelling clue – the sculpture smelled exceedingly like a Crayola crayon! I did some research about what type of materials are in a Crayola crayon and what they are soluble in, and I did some solubility tests on my wax samples.  This is what they looked like through the microscope:

Evernote Camera Roll 20160418 121532   Evernote Camera Roll 20160418 121531

I even took a video to document the solubility to show to my advisor, Sophie, when we discussed the treatment:


We decided on a solvent system that we thought checked all the boxes (dissolved the wax, didn’t dissolve the plaster and was safe to use), and I got to work:


Thankfully, everything seems to be going as planned! Poultices can be really tricky to use successfully and sometimes you end up just moving a stain back and forth through a medium instead of removing it; and it can be slow going.  Here is a poultice turning pale yellow as the solvent evaporates:


And this is the bust partway through treatment, next to a picture of the sculpture before I started removing the wax:

_MG_5398     IMG_8660_ed

While there’s still some work to be done, it’s exciting to see the wax slowly disappear!


Megan Narvey


Comments are closed.