Invisible Writing Made Visible 

G P O H I S T O R Y T A L K 

M A Y 2 9 , 2 0 1 3 

J A M E S C A M E R O N 
Invisible Writing Made Visible 

 During the Second World

War, from late 1942 to 

mid-1946, the United States 

experienced an

unprecedented influx of 

almost half a million German, 

Italian, and Japanese 

prisoners of war. They were in 

a position to damage the war 

effort through attempts to

escape and, based on 

experience from World War I,

the possibility of espionage. 
Invisible Writing Made Visible 

 Since the Geneva Convention 

mandated that prisoners of 

war could write home, 

significant information on 

prisoners’ location and 

activities could be transmitted 

through the use of invisible 

inks made from such common 

substances as lemon juice, milk, washing soda, baking soda, starch, even urine. The 

War Department turned to

GPO’s paper chemists for an 

answer. 
Invisible Writing Made Visible 

 GPO’s Division of Tests 

and Technical Controls 

was set up in the 1920s 

to monitor and 

improve the quality of 

paper, inks, adhesives, 

typemetal, and other 

vital components of 

printing and binding 

work. 
Invisible Writing Made Visible 

 After extensive tests, 

GPO’s experts developed

a paper base with a

silicate or clay coating. The coating contained a

powder or dyestuff that 

would react to moisture, 

or any acid water 

solution, by turning

green. The paper was 

called Sensicoat. 
Invisible Writing Made Visible 

 Sensicoat’s heavy 56-pound weight and high cost 

were negative factors, so GPO developed a lighter, 

uncoated, and more economical paper, Analith. 

When it went into production, secret messages to the 

Axis were greatly reduced. 
Invisible Writing Made Visible 

 German intelligence noted the change and 

took action. American 

sensors began to notice

something very

interesting about

packages of food and 

clothing addressed to 

German prisoners as 

1944 passed its halfway mark. 
Invisible Writing Made Visible 

 A small amount of 

putty-like material, 

about the size of a 

kitchen match head, 

began to turn up in 

various places of 

concealment. Repeated 

tests showed that the 

material was a dry ink. 
Invisible Writing Made Visible 

 After several conferences 

with the Bureau of 

Censorship, GPO 

chemists began work on

a new paper, bearing in

mind that it would have 

to retain its sensitivity to 

fluids as well as add 

sensitivity to dry inks. 

The result was a coated 

sheet with both. 
Invisible Writing Made Visible 

 By 1945, more than 29 million

sheets of the new stationery had been ordered at $1.04 per thousand. GPO had blocked a 

potentially dangerous flow of 

information to America’s 

enemies. It was an 

achievement shrouded in 

wartime secrecy, but one 

gratefully acknowledged by those who knew about the 

technical challenges. 
Invisible Writing Made Visible 

A D A P T E D F R O M A N A R T I C L E B Y J A M E S C A M E R O N IN 

K E E P I N G A M E R I C A I N F O R M E D : T H E U N I T E D 

S T A T E S G O V E R N M E N T P R I N T I N G O F F I C E : 1 5 0 

Y E A R S O F S E R V I C E T O T H E N A T I O N 

H T T P : / / W W W . G P O . G O V / F D S Y S / P K G / G P O -

K E E P I N G A M E R I C A I N F O R M E D / P D F / G P O -

K E E P I N G A M E R I C A I N F O R M E D . P D F 
Invisible 

Writing Made 

Visible 

 Photos of the Division of Tests 

Technical Controls are from the 

GPO Photo Collection 

 Images of WWII posters are 

courtesy of Northwestern 

University Libraries