Chemical cenzorship

Occasionally letters mailed between  Croatia and Kroatische Ausbildungs brigade carry marks of chemical censorship which was performed by German censors who were looking for hidden texts written in invisible inks. Croatian censors didnt posses  materials for chemical censorship.

Chemical censorship was fully developed during WWI, when mail circulated between warring sides via Netherlands and Switzerland. During both wars people tried to send  secret messages to the opposing side written in invisible ink. Ordinary materials like: milk , urine , starch solution or citrus fruits juices were used as well as specially developed invisible inks made by military and intelligence agencies. During WWI Germans used invisible ink such as lead-acetate hidden in toilet water and soap made from potassium ferrocyanide but they were discovered by the french scientists using electrolysis with silver nitrate or  were detected  with solutions of silver , like solution Protargol( silver proteinate). As French developed a way to detect German invisible inks they developed their own invisible inks , one of them  needing 4 developing reagents in set order, and it  was so successful that it has not been made public yet , over a 100 years later.

In 1941 Germans spies caught  in USA and Bermuda used diluted solution of Pyramidon (Aminophenazone) which was antipyretic readily available in any Pharmacy  and is still in market  today. In wasnt detected using modern kits of the time but by using old fashion  iodine solution.
 Such readily available but hard to detect compounds were best candidates for invisible inks.

In my opinion on letters from Kroatian Ausbildungs brigade mailed to Croatia  German censors were looking for simple ordinary materials and not for special inks.
On letters, often blue lines of various intensity are visible which  are most probably from diluted solutions of copper sulphate , also much more pale lines can be seen possibly from diluted iodine solutions.

Mail with chemical censorship sent to and from Croatia during WW2 is  rare.

Registered Letter sent from Stockerau-Lager 8.1943 to Podravske Sesvete ,Croatia.  Blue lines of censors solution are visible on the cover and on the page of text . Often stamps were removed by censor to check for hidden message on the back of the stamps. All mail from Stockerau Lager to Croatia was censored ,but registered mail was rare as Feldpost cards were mailed for free unlike registered mail. German censors applied regularly same censor number on the envelope and on the sheet of text , here we  can see   number 565671 on the top and bottom picture.

Cover of a registered letter mailed 5.1944 to Osijek ,Croatia from Stockerau Lager
Blue line of censors solution is visible on the back of the  cover and on the edge.
German stamp was not removed which is exception to the rule. Nice Ag censor mark , means it was censored in Wien ( Vienna)

Registered mail from Stockerau Lager to Croatia is rare .