Bone china, pure or impure?

It is permissible to use every pure vessel from the point of view of its purity, but it might be unlawful for some other reason, being made of gold or silver for example. It is forbidden to use an impure vessel except for a large amount (216 L. or more) of water or for something dry when the vessel is dry.[1]

Clay is pure and bone is considered pure if we do not know otherwise.[2] So the china you mentioned is assumed to be pure and is lawful to sell and use wet or dry. However if it is known that it is usually made from impure bones, it is offensive to use. And Allah knows best.

Beneficial axiom

If it is said, “Bones in non-Muslim lands are usually from improperly slaughtered animals, or even pigs, so they are impure and so is china made from them;” we say, “The original ruling of bone is purity, so unless we know the impurity of a specific bone (by seeing it taken from a pig for example) and the use of that bone in a specific piece of china, we act according to this origin.”

This is based on an extremely important and useful axiom, especially for those living in non-Muslim lands, known as The Origin and the Likelihood or Preponderance. It says that anything which is initially pure, but usually affected by filth, is given the ruling of purity based on its original purity. Imam Ramli (Allah have mercy on him) said in Nihaya:
If filth (najasa) is the preponderance in a thing and the origin of it is pure (such as the clothing of: an alcoholic, those who religiously use filth, the insane, children, and butchers), then it is judged pure acting on this origin. [This is the case] even if the custom typically differs, such as the use of dung in making ceramic vessels [which was common in Ramli’s time] . . . That which is generally a problem is also judged to be pure, such as: the sweat of animals, their drool, children’s drivel, and Jukh ([a substance that] was commonly made using lard), and the like of these things. And among the reprehensible innovations is washing: a new garment, wheat [which was threshed by cattle which would sometimes urinate], and the mouth after eating and the like.[3]
An example of washing the mouth after eating is washing it after eating bread because it was cooked in a ceramic oven of that time which might have been made using dung. There are many other examples of this, such as: the dishes of those who eat pork, soaps, and cosmetics that could be made using vegetable or animal byproducts and we do not know which was used. Shaykh Ibn Hajar (Allah’s mercy be upon him) also mentioned this axiom in Tuhfa and sighted as an example Syrian cheese which tended to be made using swine rennet. He said: “And cheese from them came to him [the Prophet], Allah bless and grant him peace, and he ate from it and did not ask about this.”[4] So avoid the hairsplitting of those afflicted with unfounded misgivings and make presence of heart in worship according to the sunna of our pure and purifying Prophet your concerning (Allah’s blessings and peace be upon him and his folk). And Allah knows best and He alone gives success (tawfiq).

[1] See: Ibn Hajar al-Haytami, Hawashi al-Sharwani Wa Ibn Qasim al-Abbadi ‘Ala Tuhfah al-Muhtaj Bisharh al-Minhaj (Beruit, Dar Ihya’ al-Turath al-‘Arabi), 1:117.
[2] Shaykh Ibn Hajar (Allah have mercy on him) said in Tuhfa: “If hair or the like was doubted about, is it from an edible animal or other than it, or did it come from a living or dead animal; then it is pure because the initial ruling of the like of hair is purity. The analogy is that bone is like this as well and the author of al-Jawahir said this explicitly” [ibid 1:300].
[3] Muhammad Ramli, Nihayah al-Muhtaj ila Sharh al-Minhaj (Beruit, Dar Ihya’ al-Turath al-‘Arabi), 1:101.
[4] Ibn Hajar al-Haytami, Hawashi al-Sharwani Wa Ibn Qasim al-Abbadi ‘Ala Tuhfah al-Muhtaj Bisharh al-Minhaj (Beruit, Dar Ihya’ al-Turath al-‘Arabi), 1:308.