One can notice a white paste like thing applied on the face of the Moolavar, Sri Venkatachalapathy - Tirumala. The white paste is nothing but the Pachchai Karpooram (One kind of Camphor). There is a sweet and beautiful story for that.
Sri Ramanujar established various madams and allotted His sishyas in variuos places to spread Sri Vaishnavam and about Sriman Narayanan. One among his sishya is Sri Ananthan Pillai. And He was given the Nandavana (flower park) Kainkaryam of in Tirumala, from where the flowers are dedicated to the Lord.
Likewise, one day Ananthan Pillai was collecting flowers from Nandavanam. He wanted to collect the flowers so quick, so he wanted his wife to help him out. But at the same time, his wife was expecting a baby and she is not in a position to do much work and give assistance to him.
On seeing the condition of his wife, the Perumal appeared as a small boy and helped Ananthan Pillai's wife. Inspite of carrying, Ananthan Pillai was a bit surprised how can she work so fast as equivalent to him. So, he followed his wife and at last he found a small boy helping her out. On seeing the boy, he got angry angry as he was interfering with hi Kainkaryam to the Lord (work dedicated to the Lord). So, he struck the boy with a spade (a kind of axe) and the boy was wounded on his chin. At the same time, the boy disappeared from that place.
The next day as a regular routine, Ananthan Pillai went towards the temple to dedicate the garland to the Perumal and was surprised to see the blood coming out from the Perumal's chin. After that he realised his fault and fell down to the feet of the Perumal and wept to forgive him. And finally to stop the blood, he applied the Pachchai Karpooram on his chin to stop it. From then, as the devotion of Ananthacharyar, the Perumal continued to wear Pachchai Karpooram.
This is the story why the Pachchai Karpooram is applied on the Chin of the Perumal. Even today, the spade (crow bar) used by Ananthan Pillai to dig the tank is found on the rightside of Gopura Vaasal (Entrance) of Tirumala temple.