Acid Etching to Recover ID Stamps
Camaro Research Group
16 Apr 2000

If you use this procedure, please post a note on the CRG Forum
with your impressions and results.  Please heed the safety warnings
and do not attempt this if you are inexperienced in handling hazardous
materials.

*****
I tried the acid etch, it looks like you can make out the number T0419JL.
The T04***L for certain the 19J are faint. Thanks for the tip on the acid etch.
Steve 
*****

The technical standard for metal etching is covered by ASTM
(http://www.astm.org/) E340, Standard Test Method for Macroetching
Metals and Alloys.  The ASTM standard contains background and details
not included in this CRG draft.  We strongly encourage anyone
considering metal etching to obtain a copy of this either from the
website (for a fee) or from your local library in Volume 3.01 of the
ASTM Book of Standards.

THE PROCEDURE INVOLVES USE OF ACID, AND THERE ARE OBVIOUS SAFETY
ISSUES ASSOCIATED WITH SUCH MATERIALS.  USE SAFETY GLASSES AND
ACID-RESISTANT GLOVES, READ THE SAFETY PRECAUTIONS ON THE CONTAINERS.
IF DILUTING - ADD ACID TO SOLVENT RATHER THAN SOLVENT TO ACID, BE
CAREFUL OF FUMES, DO THIS IN A VENTILATED AREA, ETC.

The theory, for those interested, is that the cold-work of the
stamping process changes the metal microstructure locally around the
imprint, sufficiently so that there is a resulting difference in
resistance (increase or decrease) to acid attack in the portion that
has been cold-worked.  If the original stamp is removed, the material
underneath retains this change in microstructure, which is not visible
to the naked eye (until being revealed through etching).

There is a limit to how much material can be removed during the
decking process before etching will not reveal the original stamp.
Obviously the more material removed, the less distinct any characters will be.

For illustration, here are the approximate steps for cast iron:

1) Prepare or purchase etching solution.  There are numerous types of
etching solutions.  The "general purpose" solution in the standard for
iron and steel is a 50/50 mix of concentrated hydrochloric acid and
water.  An easily obtainable close approximation of this is swimming
pool muriatic acid, available from swimming pool and home improvement
stores, usually already in a diluted form.  (Note: a 160F-180F
solution is suggested in the ASTM standard test method, but we
strongly recommend avoiding use of a heated acid solution in a
hobbyist garage. A room-temperature solution will work - but will take
a little longer.)

2) Remove all grease, dirt, and paint from area to be etched.

3) Mask off portions of the casting that you don't want etched. You
don't want the etching solution washing down your cylinder bores!

4) Clean the pad with solvent and/or cleaning solution. "Any grease,
oil, or other residue will produce uneven attack."

5) Saturate a cotton ball held in tongs with the etching solution,
uniformly swabbing the area of interest. Wait for the "structure to be
developed" (characters to appear). Since the estimate development time
is 15-30 minutes you may have to repeat the swabbing periodically to
keep the surface uniformly wet with etchant.  When the characters
become visible or it becomes obvious that they won't, you can stop.

6) Rinse the casting with hot water to remove the etchant.

7) If necessary, "desmut" the surface by vigorous scrubbing with a
vegetable fiber brush.

8) Blow the casting dry with compressed air.