[MassHistPres] Cast Iron Restoration

[Matthew B. Bronski]

A couple comments on the prep and the coating system:

Regarding the silica blast to remove old paint, whether or not this erodes original detail in the cast iron (and whether or not it is good or poor preservation practice) would depend on the fineness of detail, and the hardness, particle size, shape, distance of application, and particle speed of the microabrasive (silica in your case). Silica is quite hard, and has an angular particle size, which  tends to make it more aggressive than other micorabrasive cleaning materials.  I'd be quite wary of silica blasting a cemetery marker, which I assume has some fairly fine lettering or detail.    At a minimum, your spec should require for mock-ups of various candidate cleaning materials and procedures on a less sensitive area to determine appropriate materials and procedures for cleaning without damaging the artifact.  You should include softer microabrasives (e.g., walnuts shells, baking soda) in your mock-up spec, and probably chemical cleaning too, to determine the best approach.   All cleaning and coating approaches should avoid the use of water, to avoid flash corrosion (e.g., if you try chemical cleaning methods, and for any coatings, they should be solvent-borne, not water borne)

Regarding the coating, the system your spec calls for is a zinc-rich primer, with a "barrier"  finish coat.  This is a common approach for coating mild steel (but you have other options with cast iron).   The principle behind the zinc-rich primer intends to put a less noble metal (zinc) on the cast iron, so the zinc will corrode, not the cast iron.  This is the same principle used in galvanizing.   If you are going to use a zinc-rich primer or cold-galvanizing compound, you really need to go to bare "white" metal.  That's probably why they propose the silica sandblast, which may be aggressive and may erode detail.

 The underlying logic behind a barrier top coating is to exclude any and all moisture.  It intends perfection.   A barrier coating is the common and necessary approach on mild steel that is exposed to the weather, because of the low carbon content and the progressive way in which it corrodes, expands, and flakes if left unprotected.   You can also take this approach with cast iron, but it is not necessary, and you can take an entirely different approach.  

Because of the higher carbon content in cast iron than mild steel (common contemporary structural steel), cast iron will form a thin protective layer of corrosion that is self-arresting, (it will not continue to corrode).  That is why you see cast iron manhole covers left exposed to the weather, out in the rain and snow all the time, sitting in the roadways, that are not highly corroded or flaking like steel.  They have a thin layer of corrosion that self-arrests.   So another viable approach with cast iron is to either leave it alone and let it weather (no primer or top coat at all), or wire brush it and oil it with a mineral oil (like baby oil).  The latter gives it a dark brown color that looks almost like statuary bronze (not as shiny, but similar color brown). The maintenance on the oil finish is easy, and almost anyone can wire brush and oil it later, so it's typically less costly to maintain than an industrial grade barrier paint coating.   You still see some beautiful 19th century cast iron fences that are unpainted, or simply oiled.  

Do you know if the cemetery markers were originally painted entirely, or if the paint was added later?   Or if only the letters were painted?  So many things that were painted white in the early twentieth century, when Colonial Revival fever took hold.  I've done a somehwat similar paint system to the one you describe on cast-iron spandrel panels on an early twentieth century building, where the cast iron was quite thick and had no delicate detail, and it was originally painted, and where the salmon-orange color was important to the original design (it matched the color of some glazed terra cotta trim on the building).  But if color is not an essential part of the original design, oiling bare cast iron can provide an excellent and period-appropriate option for cast iron. 


Matthew B. Bronski
Staff Engineer/Architectural Designer
 
Simpson Gumpertz & Heger
41 Seyon Street, Bldg. 1, Suite 500
Waltham, MA  02453
781.907.9264 direct 
781.907.9009 fax
www.sgh.com

-----Original Message-----
From: masshistpres-bounces at cs.umb.edu [mailto:masshistpres-bounces at cs.umb.edu] On Behalf Of Jonathan Feist
Sent: Saturday, October 27, 2007 4:16 PM
To: Massachusetts Historical Preservation interests
Subject: [MassHistPres] Cast Iron Restoration

Hello,

There is a proposal afoot to restore the cast iron markers in the Harvard
Shaker cemetery. The method they want to use is:

³We will strip all old paint and rust off with a fine silica sand blast.
Then we will etch the parts with an iron phosphate wash followed by a
stabilizing outgas to force any impurities out of the metal. We will apply
2­4 mils of zinc rich epoxy powdercoat primer followed by 2-4 mils of gloss
white TGIC polyester powdercoat finish coat.²

Does anyone know whether this approach is considered good preservation
practice?

--Jonathan



===============================================

Jonathan Feist, Chair
Harvard Historical Commission
978-772-4864 (home)
617-747-2148 (Berklee office)

Preserve Historical Harvard, MA:
http://www.jonathanfeist.com/Pages/HarvardPreservation.html


New Article: The Habit of Pie
http://www.harvardpress.com/Features/TheHabitofPie/tabid/630/Default.aspx


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