Distinguishing Official Stamps printed on "thin hard paper" from those printed on "soft porous paper".
Howard M. Relles
rellesh@yahoo.com

An attempt to unambiguously distinguish Official Stamps printed on "thin hard paper" from those printed on "soft porous paper" is described below.  It is based on the expected phenomenon that soft porous paper will wick water faster than hard paper.  (This is a third group of stamps whose identification is being augmented using the paper's wicking behavior; see the effort to distinguish Bank Note stamps from one another:  http://www.rellesart.com/Wicking91604.html  and  http://www.rellesart.com/Wicking92804.html  ).

Twenty-nine Official Stamps were completely freed of all remnants of hinges or residual gum (from hinges)(each of the "mint" examples in this study were already without any gum) in warm (about 140F) water.  After drying and allowing them to remain at 80°F (60% relative humidity) for at least 24 hours, I still could not tell by feel which (if any) were printed on "thin hard paper" and which (if any) were on "soft porous paper".  They all felt too similar to allow for a definitive identification.

I then carefully placed a 3.5-4.5mm drop of water on the back of each stamp on a horizontal surface and allowed it to form its mostly circular wicking pattern.  The maximum wicking distance (the largest diameter of the mostly circular wicking pattern) was measured every minute for 5 minutes taking care not to disturb the remaining drop or the wet area.

Results:
Considering the 125+ year history of each stamp, it was surprising to see that all 29 stamps fit into two distinct wicking categories: (a) little or no wicking (distance/time: 0.1 ± 0.1 mm/min.) and (b) considerable wicking (distance/time: 1.3 ± 0.2 mm/min.).  I've assigned the little or no wicking examples to varieties printed "thin hard paper" (1873, Continental Bank Note Co.) and the considerable wicking examples to those printed on "soft porous paper" (1879, American Bank Note Co.).  Six of the stamps used only are known (Scott's 2004 U. S. Specialized Catalog) in the 1873 variety and each of these was found to be of the little or no wicking type.  The experimental data is shown in the following table.

Wicking Distances (mm) for OFFICIAL STAMPS vs. Time (min.)

Test
Stamp
 Partial
Description
 0
min.
 1
min.
 2
min.
 3
min.
 4
min.
 5
min.
 Slope
mm/min.
 Stamp I.D.
(Scott's #)
 Not this Stamp
(Scott's #)
A
 3¢ Agriculture Dept., Used
 3.5
 3.5
 4.0
 4.0
 4.0
 4.5
 0.2
 O3
 O95
B
 3¢ Interior Dept., Used
 3.5
 3.5 3.5 3.5 3.5 3.5 0.0
 O17
 O98
C
 3¢ Interior Dept., Used 4.0
 4.0
 4.0
 4.5
 4.5
 4.5
 0.1
 O17
 O98
D
 3¢ Interior Dept., Used 4.0
 4.0
 4.0
 4.5
 4.5
 4.5
 0.1
 O17
 O98
E
 6¢ Interior Dept., Used
 3.5
 3.5 3.5 3.5 3.5 3.5 0.0
 O18
 O99
F
 12¢ interior Dept., Used
 4.0
 4.0 4.0 4.0 4.0 4.0 0.0
 O20
 O101
G
 24¢ Interior Dept., Used
 4.0
 4.0
 4.5
 5.0
 5.5
 5.5
 0.3
 O22
 O103
H
 30¢ Interior Dept., Used
 3.5
 3.5 3.5 3.5 3.5 3.5 0.0
 O23
 ---
I
 6¢ Navy Dept., Used
 3.5 3.5 3.5 3.5 3.5 3.5 0.0
 O38
 ---
J
 3¢ Post Office Dept., Used
 3.5 3.5 3.5 3.5 3.5 3.5 0.0
 O49
 O108
K
 3¢ Post Office Dept., Used
 3.5 3.5 4.0
 4.0
 4.0
 4.0
 0.1
 O49
 O108
L
 3¢ Post Office Dept., Used 4.0
 4.0 4.0 4.0 4.0 4.0 0.0
 O49
 O108
M
 1¢ Treasury Dept., Used
 4.5
 4.5 4.5 4.5 4.5 4.5 0.0
 O72
 ---
N
 2¢ Treasury Dept., Used
 3.5
 3.5 3.5 3.5 3.5 4.0
 0.1
 O73
 ---
O
 3¢ Treasury Dept., Mint NG
 3.5
 3.5
 3.5
 4.0
 4.0
 4.0
 0.1
 O74
 O109
P
 3¢ Treasury Dept., Used
 4.0
 4.5
 5.0
 5.0
 5.0
 5.0
 0.2
 O74
 O109
Q
 3¢ Treasury Dept., Used
 4.0
 4.0 4.0 4.0 4.0 4.0 0.0
 O74
 O109
R
 6¢ Treasury Dept., Mint NG
 3.5
 3.5 3.5 3.5 3.5 3.5 0.0
 O75
 O110
S
 12¢ Treasury Dept., Used
 3.5 3.5 3.5 3.5 3.5 3.5 0.0
 O78
 ---
T
 15¢ Treasury Dept., Used
 4.0
 4.5
 4.5
 5.0
 5.5
 5.5
 0.3
 O79
 ---
U
 90¢ Treasury Dept., Used
 4.0
 4.0 4.0 4.0 4.5
 4.5
 0.1
 O82
 O113
V
 10¢ War Dept., Mint NG
 4.0 4.0 4.0 4.0 4.0 4.0 0.0
 O88
 O118
W
 30¢ War Dept., Mint NG
 4.0 4.0 4.0 4.0 4.0 4.0 0.0
 O92
 O120
X
 3¢ Interior Dept., Used
 4.5
 7.5
 9.0
 10.0
 10.5
 11.5
 1.4
 O97
 O16
Y
 30¢ Treasury Dept., Used
 3.5
 5.5
 7.5
 9.0
 9.5
 10.5
 1.4
 O112
 O81
Z
 1¢ War Dept., Used
 4.0
 6.5
 7.5
 8.5
 9.0
 9.5
 1.1
 O114
 O83
AA
 2¢ War Dept., Used
 4.0
 6.0
 8.0
 9.5
 10.0
 11.0
 1.4
 O115
 O84
BB
 3¢ War Dept., Mint NG
 4.0
 6.5
 8.5
 9.5
 11.0
 12.0
 1.6
 O116
 O85
CC
 6¢ War Dept., Used
 4.0
 5.5
 6.0
 7.0
 8.5
 9.5
 1.1
 O117
 O86