[MassHistPres] Cast Iron Restoration

[Boston Affiliates]

Matthew,

Thank you - both for the information, and for 
including the original question in your answer! 
I'm interested in cast iron projects, as I'm 
working on one in Boston's South End, and I've 
been trying to figure out who was replying to what!

Polly Harrell

At 12:01 PM 10/31/07, Matthew B. Bronski wrote:
>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



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