I would like to know something about 'dhuni' and its association with the 'Nath' tradition. Any references from some authentic resources would be helpful.
Dhuni is basically a sacred fire burned in a site and a part of fire worship. In Nath tradition sadhus (celibate monks) in their later life settle at one place and ignite and worship sacred fire in the pit using various substances which is known as Dhuni.
As you have inquired this sacred Dhuni is not a simple burning of wood . In the Nath-Sampradaya this Dhuni is a living deity and sacred is symbolic of womb of the Goddess Earth from which all universe generated, sustained, and destroyed. In other words we can say that Dhuni is tradition of fire worship. Natha yogis kept this ancient tradition of holy flame alive. A dhuni therefore represents a site of worship dedicated to Shakti in Nath sampradaya.
We can get an in depth idea about concept , procedure of Nath tradition Dhuni . from a good article from the International Nath order web site - AWAKENING THE NATH DHUNI.
Nath sampradaya yogi's in their later life settle at one place and there a sacred pit is excavated which is used to burn various substances and the fire and the smoke is called as Dhuni. These Yogis' seat in circle or sometimes alone around the fire and chant mantras and ritual instruments are also used in the worship. The substances used in Dhuni are Ghee and various aromatic substances plus wooden sticks.
A site is carefully selected and an appropriately sized cleft or pit is excavated. In homage to the womb which gave us all life, the Dhuni is shaped in like fashion. Naths are aware that it is the intention and attitude of the individual which is an all important element in the transformation of their personal perceptions. With this awareness it is the intention, attitude and will of the Natha which transforms a pit in the ground into a living embodiment of the earth goddess or Dhuni Divine. This sacred site then becomes the focal point for individual and group transformations of Awakening and worship.
Once the participants are seated in a circle around the Dhuni, the opening mantras are begun at the predetermined time. These mantras begin to establish the mood and focus the participants attention on the Dhuni.
All participants chant mantras and wield ritual instruments while the Fuel Master feeds the Dhuni to a full spectrum blaze
Ghee or clarified butter and aromatic substances are energetically but consciously offered to the Dhuni and its’ glowing embers. A fragrant and beautifully refined blaze now manifests as the Dhuni reaches spiritual ecstasy along with the blissfully frenzied expressions of the participants. The sounds of the chimpta, damaru and drum once again mix with the reverberations of exalted chanting.
Ashes from previous Dhuni rites are then used to mark the body of the participants in the ancient manner of Nath tribal tradition. Ritual instruments are used liberally at the rite, and often participants rise and dance while chanting and playing a chimpta or damaru.
The Mantras used at Dhuni rites have been passed down from Guru to Disciple in an unbroken chain stretching back to before the time of recorded history. The Pagan rites of the Nath Dhuni are ancient, but still astonishingly effective in their primal simplicity and power.
Importance of Dhuni in Nath tradition.
In Nath Sampradaya Goraksha is considered as Supreme Brahman. Lord Vishnu is believed to create this earth from ashes of Goraksha. There are several legends associated with Dhuni , which are associated with creation-maintenance-Destruction of the universe.
Another Nath Siddha tradition relates the creation of the earth from ashes from the Dhuni of the goddess Shakti. Whe Shiva promises to marry her, shakti gives shiva two handful of ash from her Dhuni . When these ashes are strewn upon the waters , the earth is created. Ina similar account Vishnu creates the earth with ashes from Gorakh's dhuni.
Symbolism behind using ash of divine dhuni
As all ascetics use sacred ash, the Nath yogi's also use it as mark (tripundra , half-moon drawn three times) on forehead as well as to rub the whole body and the hair with the ash from sacred dhuni as tradition. This ash is either mix with earth or burnt cow-dung. In Nath tradition it is believed that earth itself is created from ashes of Dhuni. Our body also will be reduced someday in the ashes like ash of Dhuni.
Vibhuti (Ashes) Several reasons are given for the use of ashes.' They signify death to the world,' and, in this case, undoubtedly refer to the burning grounds; or, they may indicate that the body must be reduced to ashes ultimately,' or they may be a sign that the Yogi has abandoned the world. Ashes protect from evil spirits as well.
Here this wikipedia article about Dhuni.
A dhuni is worshiped by spiritual intention and the kindling of a flame inside it. Suitable materials are offered to the dhuni and consumed by the heat or flame. This represents the eternal process of change and transformation on all levels of existence.
"Like a river, a dhuni is always changing. Each dhuni also has its own personality that is as much subject to moods as a person. The glow of the dhuni is both a receiver and a transmitter, and like a screen on which Rorschach-like images are projected, it delivers a code"
As the yoni is the nexus from which all manifest beings come into this world, the worship of the dhuni represents a sacred nexus for the path of return from the physical to spiritual level. This is an intentional process of inversion or return to our spiritual source. The dhuni is a sacred site and focal point for this form of spiritual exertion or sadhana.
Aside from the offering of sacred fuel to a dhuni, mantras are also offered, as well as the sounds of diverse musical instruments and ecstatic dance and gesture.
Though several cultures retain traditions of fire worship (out of which the Zoroastrianism is perhaps the most famous), a unique feature of the dhuni tradition is that it is the dhuni, the actual site itself which is considered sacred, not exclusively the fire kindled within it.
And lastly here is Ultra-slow motion video of Dhuni