Conserving a WWII Black Widow Compressor

Posted on Thu, February 4, 2016
  • by: Claire Taggart is a current graduate fellow at Winterthur/University of Delaware Program in Art Conservation and was a conservation intern with the Museum in summer 2015.

The Museum has been tackling a variety of artifact treatments through its Collections Care and Preservation Fund (CCPF) Triage Project. The goal of the project is to stabilize artifacts as they are moved from the Paul E. Garber Preservation, Restoration, and Storage Facility to state-of-the-art storage located at the Steven F. Udvar-Hazy Center in Chantilly, Virginia. The triage project addresses urgent issues affecting artifacts such as active corrosion, pest infestation, biological growth, and physical insecurities.

As a conservation intern with the Museum’s triage team this past summer, I treated objects from both the space and aeronautical collections. One of the more complex and extensive treatments I completed was on a General Electric compressor used in the turret of a WWII-era Northrop P-61C Black Widow, much like the one on view at the Udvar-Hazy Center. The Black Widow was the first of its kind, designed to intercept and attack enemy aircraft at night and in bad weather. After the war, the Museum’s Black Widow was used by the Army Air Forces for inclement weather testing, and consequently it had its top turret removed to make room for weather monitoring equipment. This compressor would have provided the turret with the charge needed to operate the automatic machine gun.

Even though researching the function of each component of this compressor was fascinating, fully grasping each mechanism was not necessary for treating it. The important considerations for treating the compressor were understanding its different corrosion types; determining if the corrosion was active and needed treatment; determining whether or not there were any hazards associated with treating the materials; and deciding how to stabilize the object to inhibit further degradation. Lastly, a support was created so that the object could be safely stored and handled after treatment.

General Electric Compressor Components

<p>Before treatment, the General Electric Compressor's components were inventoried and inspected. </p>

 Before Treatment: The compressor consists of a crankcase, low and high pressure cylinders, chromate coated mounting bracket, motor stator assembly, fan and fan guard, relay box, and detached intercooler.

Magnesium and Ferrous Corrosion on General Electric Compressor

<p>Before the General Electric Compressor was treated, there was magnesium and ferrous corrosion near I.D. plate.</p>

 Before Treatment: Detail of magnesium and ferrous corrosion near I.D. plate.


A literature review of previous tests performed on similar objects provided a useful framework for approaching the conservation treatment of the compressor. For example, like other objects treated in the conservation laboratory, the magnesium corrosion appeared in abundance on the crankcase and pressure cylinders of the compressor as white, fluffy, powdery particulates. To treat the magnesium corrosion, I applied successive grades of metal polish, being sure to clear away the polish between applications. This was followed by many hours of mechanical removal of the corrosion products under magnification using a scalpel and other small hand tools. Due to its elemental properties, magnesium is highly reactive and corrodes very quickly; some of the more deeply pitted areas seemed to regrow the white corrosion overnight!

Cleaning the General Electric Compressor

<p>Carefully cleaning magnesium corrosion under magnification on the General Electric Compressor. </p>

 Carefully cleaning magnesium corrosion under magnification.


Other corrosion types I addressed were ferrous corrosion and cadmium corrosion. The ferrous metal fittings had once been plated using cadmium, however the plating had largely failed due to corrosion of the underlying metal. It was useful to visually characterize this under long-wave UV illumination as cadmium fluoresces orange under UV light and it is considered hazardous and should be handled with caution.

General Electric Compressor Viewed Under UV Illumination

<p>Before treatment the compressor was viewed under long-wave UV illumination to show hazardous corrosion. </p>

 Before Treatment: Compressor viewed under long-wave UV illumination. Magnesium corrosion appears as bright white (crank case, cylinders, and fan mount), ferrous corrosion as dark red (motor housing and fan cage), and cadmium can fluoresce orange (nuts, screws, and bolts, as well as trace amounts hidden on the fan mount.)


In the areas where cadmium was most likely present, careful clearing (or cleaning away) was performed with deionized water before treating the ferrous corrosion. Reduction (reducing materials) was then completed using cleaning pads in multiple grades, steel wool, and a fiberglass bristle brush and disposed of afterwards in a safe manner.

General Electric Compressor Corrosion

<p>Before treatment the General Electric Compressor had an area of dark orange ferrous corrosion in need of reduction and stabilization.</p>


 Before Treatment: Area of dark orange ferrous corrosion in need of reduction and stabilization.


Once the corrosion products on the compressor had been reduced, it was necessary to protect the surfaces from further degradation. A penetrating oil and a wax coating were applied as corrosion inhibitors to all the treated areas.

GE Compressor After Conservation

<p>General Electric compressor used in the turret of a WWII-era Northrop P-61C Black Widow after conservation treatment. </p>

 GE Compressor after treatment.


As a final measure, supportive housing was assembled using archival quality materials. This ensures that handling, examination, and potential research can be done without further damaging the object.

General Electric Compressor Archival Housing

<p>To protect the General Electric Compressor, supportive housing was assembled using archival quality materials. </p>

 Constructed housing for compressor's protection.