The Germans decided to concentrate on the notes with the largest number in circulation, the £5. Samples of British notes were sent to technical colleges for analysis, which reported back that it was rag paper with no added cellulose. Naujocks and Langer realised that the paper would have to be handmade. The colour of the first samples differed from the original; the Germans had used new rags. After tests at the Hahnemühle paper factory, the rags were used by local factories and then cleaned before being used to make the paper; the colours of the counterfeit and original notes then matched. When the initial paper samples were produced, at a factory in Spechthausen, they looked identical to British notes in normal light, but looked dull in ultraviolet light when compared to the originals. Langer surmised it was because of the chemical composition of the water used for making the paper and in the ink. He duplicated the chemical balance of British water in order to make the colours match.