What Tilak or Namam does Lord Brahma have on His forehead?
The best iconography of Brahma is found in the Puranas (such as Matsya Purāṇa Chapter 260 and Agni Purāṇa chapter 49) and other iconography in some medieval texts on architecture (the most famous being Aparājitapṛcchā Chapter 214, Devatāmūrtiprakaraṇam Chapter 4, Mayamatam Chapter 36) but the tilaka of Lord Brahma was nowhere to be found except in only one book called the Mānasāra
Chapter 51 of the Mānasāra says:
रत्नाङ्गुलीयकौ हस्तौ पादं जालसरत्नकम् ललाटे वृत्ततिलकं स्याच्चन्दनागरुमेव च॥२०॥
His thumb rings and hands should have gems and so should his feet. On his forehead a circular tilaka of sandal (Chandan) and agar paste.
As per the above we can see that a Brahmaji must have a circular tilaka made of sandal and agar, akin to Shakta worshippers (give credit to a good knowledgeable friend for this statement) but the material and maybe the size, being the differentiating factor. This corresponds to the iconography of some of the ancient temples of Southern India as well as roughly the famous Brahma Temple at Pushkar. See the below pictures for the corresponding iconography.
1. Lord Brahma at Brahmapureeswar temple, Tamil Nadu:
Same Brahma idol with turmeric:
2. Lord Brahma at Pushkar Temple, Rajasthan:
Note: At Pushkar, while decorating the Lord, the idol is given a tripundra or occasionally an Urdhva Pundra too. However as one can see the original idol (without alankara) during abhishekam in the image has a circular tilaka.
3. Other Miscellaneous Iconographies:
From Tamil Nadu:
From Aihole, Karnataka:
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