Originally, most Advaitins had Vishnu as their Ishta Devata, but nowadays most Advaitins have Shiva as their Ishta Devata. Kanchi Mahaperiava, the former Shankaracharya of the Kanchipuram, explains the reason for the shift, in an interview found in the end of the book "History of Early Vedanta Philosophy":
Some Shankaraites are Vaishnavas; they bear the mark in three white vertical ash lines. But many others are Shaivas; they bear the mark of their own. Some other bear no mark. After Shankara there came many Tirthankaras: Ramanuja, Madhva, Vallabha, etc. They were mostly followers of the Vishnu cult. So, as a counteraction to them, most of the Shankaraites became Shaivas. This is why we find so many Shiva worshippers among the Shankaraites. I myself am just indifferent to these distinctions. (The Shankaracharya bore only one white horizontal line on the forehead then.) For example, when a devout believer prostrates all his body on the ground, I confer benediction upon him, touching him on the forehead with my hand, saying: "Narayana!" On that occasion I have the god Vishnu in mind. But when I observe the Linga worship, I am a worshiper of Shiva. I am neither Shaiva, nor Vaishnava.
I'm interested in the part in bold. I've never seen an Advaitin wear an Urdhva Pundra made of ash before. For those who don't know, an Urdhva Pundra or Namam is a symbol Vaishnavas wear on their forehead. It consists of three vertical lines and is usually made of schistose mica (Thiruman) along with either turmeric or sandal paste (Srichurnam), in contrast to the Shaivite Tripundra which consists of three horizontal lines and is made of Vibhuti or white ash. My question is, what is the origin of Advaitins wearing an Urdhva Pundra made of ash?
Is it described in any scripture? Or is it just meant to be a compromise between the Vaishnava Urdhva Pundra and the Shaivite Tripundra?