Some 27 secret letters written in urine from the women's concentration camp in Ravensbruck with information about medical experiments conducted on prisoners have been gifted to a small museum in Poland where they will undergo preservation work.The collection of the letters, gifted to the "Under the Clock" Martyr Museum in Lublin, in eastern Poland, came from the family of one of its authors, Krystyna Czyz-Wilgat.Texts were written using a thin linden stick. As a result of an acid reaction with the paper, urine lost its color after a short time and then became invisible."The next letters were read properly, how? The letters were ironed and the writing turned brown," explained Barbara Oratowska, the curator of the museum.In order to read out the encrypted message it was necessary to heat up the letter. In the first letter that was sent there was a clue that the next letters will be written with urine.Thanks to the encrypting messaging, the list of 74 women from Lublin that were subjected to medical experiments by Nazi doctors in Ravensbruck, such as being injected with gangrene to test new drugs, eventually became public knowledge."There, in the German camp the experiments were carried out by German doctors with professor titles," Oratowska said."Even though there were broad reports about the Auschwitz camp, on Ravensbruck there were was little information released. And only those female Poles were the ones who conveyed this information. That is why these letters are such a valuable material and historic evidence," she added.The letters, sent between 1943-1944 from Ravensbruck, a German concentration camp 50 miles (80km) north of Berlin, had messages written in urine between the lines and in the margins of the letters to families, which were censored.Oratowska said some of the letters were in a poor condition and needed preservation work. It was unknown whether and if they will be on a public display.Between 1939 and 1945 some 130,000 - 132,000 female prisoners passed through the Ravensbruck camp, with a third of them being Polish.Thanks to the letters, the experiments in Ravensbruck were known abroad before the end of the war in 1945.After Germany's defeat, 20 doctors from several camps were tried as war criminals by a United States military court in the first of the postwar Nuremberg trials. Seven received death sentences and seven others got long prison sentences.