Many people use olive oil for Shabbat candles (which is, indeed, the preferred manner of observing the Misva), pouring the oil into glass cups and then placing a wick. The question was raised concerning the permissibility of adding some water to the bottom of the glass cup. For example, if a woman does not have a large amount of oil to use, she might want to pour some water to lift the oil and wick so that the wick protrudes above the rim of the cup, making it easier for her to light it. Assuming, of course, that this is done before Shabbat, would this be permissible?
This issue is discussed by the Shulhan Aruch (Orah Haim 265:4), who writes that if one’s intention when pouring the water into the oil cup is not to cause the flame to be extinguished sooner, then this is permissible. As long as one does not pour the water for the purpose of hastening the extinguishing of the flame, he may add the water, and thus it would be permissible to add water for the sake of lifting the wick. The Rama (Rabbi Moshe Isserles of Cracow, 1525-1572) disagrees, citing authorities who allow adding water even for the purpose of having the fire extinguish sooner, since the extinguishing is done indirectly. Regardless, even according to the Shulhan Aruch, it would certainly be permissible to add water to the oil cup for the sake of elevating the wick.
Hacham Ben Sion Abba Shaul (Israel, 1923-1998), in his work Or Le’sion (vol. 2, 18:17), addresses the question of whether this Halacha applies also to one who adds water in order to prevent the glass cup from becoming very hot and cracking. He cites some authorities who claimed that adding water for this purpose is akin to adding water for the purpose of causing the flame to extinguish, and is thus forbidden. Hacham Ben Sion, however, disagrees, arguing that these cases are very different from one another. In his view, as long as the intent is not for the candle to extinguish sooner, one may add water, even for the purpose of maintaining the glass’ temperature. He adds that we may also take into account the Rama’s ruling, that regardless of one’s intent it is permissible to add water to the cups. The final Halacha, then, is that one may add water before Shabbat to the oil cup in order to avoid cracking. By the same token, it would be permissible to add water in order to prevent the glass from blackening.
Summary: One may add water before Shabbat to the glass oil cups of the Shabbat candles, such as to prevent the cups from cracking or blackening, as long as this is not done for the purpose of having the candles go out sooner.