What is the meaning of the camphor fount and what is the cup tempered there?

This is the purpose underlying the symbolic words of the Manifestations of God. Consequently, the application of the terms "sun" and "moon" to the things already mentioned hath been demonstrated and justified by the text of the sacred verses and the recorded traditions. Hence, it is clear and manifest that by the words "the sun shall be darkened, and the moon shall not give her light, and the stars shall fall from heaven" is intended the waywardness of the divines, and the annulment of laws firmly established by divine Revelation, all of which, in symbolic language, have been foreshadowed by the Manifestation of God. None except the righteous shall partake of this cup, none but the godly can share therein. "The righteous shall drink of a cup tempered at the camphor fountain."
     Bahá'u'lláh, Kitab-i-Iqan, p. 41 [emphasis added]

        Observant Bahais can be culturally and linguistically handicapped when it comes to wine metaphors. I had a devil of a time coming to grips with the terminology and referents of medieval wine convivia in Persian and Arabic poetry, but I think I can help some with camphor.

        Wine in the ancient world was infrequently imbibed raw or undiluted. There was cooked or mulled wine (the French term for cooked "cuit" has come into wine terminology as "cute") and wine was usually mixed with water or some other flavoring. The word for wine used in the 11th century Qaabuus naameh is seyaki, meaning three parts to one (probably one-third water or some water-dissolved flavor such as rosewater, ginger, etc.).

        The rituals of wine drinking were also quite symbolic; in Herodotus you will find a tale about how the Scythians were allowed to join the drinking circle only after having slain their first man in battle. The Persian kings drank heavily according to Herodotus, and generally took their decisions of state while drunk, to be ratified in the morning when sober. The royal wine banquet among the Persians remained an elaborate affair, all the way through the Islamic period.

        Camphor (probably originally from Sanskrit kappuura, as the two trees which go by this name originated in Indochina) is one of the spices that was apparently mixed into wine, at least among the Arabs (chemical composition of it is C10-H16-O). It has a unique and pleasant fragrance and a bitter taste. It is white in color, and could be reduced to powder but also apparently produced whitish-transparent globules. It was widely used as a cosmetic in the ancient Near East. In Persian folklore and medicine, it was believed to be an anti-aphrodisiac, though I'm not sure that this meaning has anything to do with the Camphor Fountain at hand. Herbs have different virtues attributed to them by different cultures at different times, and this may have been a digestive aid. I think it is still considered to have sedative properties and in ancient medicine, inhaling its fragrance was considered to relieve headaches.

        While the analgesic properties may be relevant (see below), I think the semiotic range of Camphor applicable in poetry and literature is less medicinal and more in the realm of whiteness/beauty, sweet fragrance (to complement delectability) and coolness/refreshment. Persian forms many compounds with Camphor, which due to its whiteness (and the association with snow) and perfume, is metaphorically combined with verbs of raining, shedding, casting and sifting. It was kept in special vials (kaafuur daan) and the word was used in compound adjectives to refer to white clothes, white skin, white hair and there is also a word for camphor-eater.

        In scripture, the Hebrew word Kopher occurs in the Song of Solomon [Canticles]: 1:14, 4:13), though this may refer rather to the Henna plant than camphor or camphire. The locus classicus for the image of the Camphor Fountain is, naturally, the Koran, where the word Kaafuur occurs only once, in Suurat al-insaan (Sura 76). Some classical dictionaries give its meaning as palm frond or calyx, specifically of an Indian tree (an exotic and therefore expensive spice), but I think the specific camphor plant is envisaged:

      v5: Inna al-abraara yashrabuuna min ka'sin kaana mazaajuhaa kaafuuran
      v6: `aynan yashrabu bihaa `ibaadu'llaah yufajjiruunahaa tafjiiran

The righteous drink from a cup [here, a measure of wine, not the actual vessel] mixed with camphor
A fountain [spring--`ayn] from which the servants of God drink, digging a channel for it to gush through
        The paradaisacal imagery goes on to describe (vv12-14) the heavenly reward for the righteous, which includes a luxurious garden, silken garb, pillows upon which one can loll about protected from the sun and from chill, with overhanging shade and boughs laden with fruit. Furthermore, there will be silver vessels and crystal goblets passed around (this an allusion to ceremonial rounds of wine-drinking, vv 15-16), and they will be served in these goblets a cup (i.e., a measure of wine [I believe]), mixed with Zanjabiil from a fountain (spring) there, called Salsabiil (vv17-18).

        In modern Arabic Zanjabiil means ginger, and it was probably considered a digestive aid. More importantly, though, ginger was an exotic spice, desirable for flavoring; undoubtedly it added a certain pungency to wine (Arabs made mostly date wine, I think, though they were by no means unfamiliar with Roman, Greek and Persian wines.

        All these spiced wines were served by androgynous, unageing youths, scattered around the wassailers like white, shiny pearls, dressed in fine green silks and wavy brocades, decked with silver bracelets. God here gives them to drink a pure wine sharaaban tahuuran (Q76:19-21).

        This is evidently the same wine of Suurat al-waaqi`a (Sura 56), proffered once again by androgynous youth (the famous dark-eyed Houris) circling around the reclining denizens of paradise, pouring out cups of spring-derived intoxicants into goblets from pitchers. Despite all this drinking, though, "they will not experience an after ache nor will they suffer intoxication" (56:19). Perhaps here the camphor's supposed analgesic properties are relevant.

        So, in short, Bahaullah has alluded directly to a verse of the Koran, one in which the wine from a gushing spring in paradise, probably identical with the one named Salsabil elsewhere in the Koran, has been tempered with camphor, a sweet fragrance adding perhaps a slightly pungent but refreshing tang to the wine, perhaps even giving the drink a whitish hue (the color of milk and purity), and staving off a hangover.