Madder lake is reported in literature to have a characteristic pinkish-orange UV fluorescence, both in microscopy cross-sections imaging  and UV Fluorescence Photography . Madder lake is the only historical red lake known for showing UV fluorescence. So, its UV fluorescence allows its identification. Though, I never could observe any UV fluorescence in my madder lake samples. So, as usual, I made some little experiments.
A lake is a transparent pigment made by precipitating an organic dyestuff onto an inert substrate, traditionally alum. Madder lake dyestuff comes from the roots of Rubia Tinctorum, a plant growing in Europe, Middle East and Asia. As red lakes for painting in Europe were more common those from scale insects but in the nineteenth century the use of madder lake was encouraged for trading reasons. There is plenty of literature on lake pigments  (chemistry, historical use, production methods and identification).
Not one but many Madder lakes
Madder lake contains alizarin, purpurin and pseudopurpurin in different proportions and pseudopurpurin and purpurin are the UV fluorescent component . The lack of fluorescence in madder lake arises from the absence of these last ones. This is due to different methods of production. It is reported  that lakes prepared by gentle alkaline treatments contain more pseudopurpurin than lakes prepared using more strong conditions. Madder lake found in paintings between 1875 and 1917 is highly fluorescent : Edgar Degas’s Portrait of Elena Carafa (1875), Claude Monet’s Flood Waters (1896), The Water-Lily Pond (1899) and Irises (1914 – 17). Indeed, those lakes were found to contain high percentage of pseudopurpurin.
Kremer Pigmente GmbH Madder lakes
I bought one sample of madder lake from Kremer Pigment, for my historical pigments collection but it didn’t show any UV fluorescence. Actually, Kremer carries 9 madder lakes with different hues, produced with different processes. When I exposed my disappointment to Kremer they were so kind to send me samples of all their 9 madder lakes.
Multispectral Imaging of Madder lake
The set of 9 madder lake samples is composed of (Kremer product number) 37202, 37203, 37217, 37218, 372051, 372052, 372057, 372058, 372142. I painted them in linseed oil on a preparation of terra Pozzuoli, a red ochre with no UV fluorescence.
So, if you want show your conservation students madder lake UV fluorescence go for Kremer madder lake 373051.
Thanks again to Kremer Pigments for the Madder lakes samples.
 J. Kirby, M. Spring, C. Higgit “The Technology of Eighteenth– and Nineteenth-Century Red Lake Pigments” National Gallery Technical Bulletin, Volume 28, 2007.
 J. Kirby, M. Spring, C. Higgit “The Technology of Red Lake Pigment Manufacture: Study of the Dyestuff Substrate” National Gallery Technical Bulletin, Volume 26, 2005.
 “Artists’ Pigments: Volume 3: A Handbook of Their History and Characteristics” National Gallery of Art, Washington; 3 edition (1998). Elisabeth West FitzHugh (editor).