Short answer
Yes, according to most scholars and halal certifiers. Cream of tartar (potassium bitartrate, E336) forms as crystals in wine barrels, but it is dissolved, purified and recrystallised into a white powder with no wine or alcohol left. In the Hanafi school this counts as a complete change (istihala), so it is halal.
Cream of tartar is halal, because the crystals have fully changed from wine into a different, non-intoxicating salt and are purified before use. Tartaric acid (E334) takes the same ruling.
The four schools agree on this.
Explanation
Cream of tartar is a white powder used in baking: it stabilises whipped egg whites for meringue, keeps sugar syrups from crystallising, and is the acid in some baking powders. It is also used in homemade play dough. On labels it may appear as potassium bitartrate or E336.
During winemaking, potassium bitartrate settles out of the wine as hard crystals on the inside of barrels and tanks. These crystals are collected, dissolved in hot water, filtered and recrystallised. The result is a pure mineral salt: it is not wine, it does not intoxicate, and nothing of the wine's nature remains.
The Hanafi school holds that a substance which changes completely into something new takes the ruling of the new thing, as when wine turns into vinegar. Cream of tartar is a clear case of this, so it is halal and pure. Some Muslims prefer to avoid it out of caution, which is a personal choice, not a requirement.
Baking powders that use other acids, such as sodium acid pyrophosphate, are also halal. For related ingredients see white wine vinegar and E-numbers.
The evidence
Quran, Surah An-Nahl 16:67
وَمِن ثَمَرَٰتِ ٱلنَّخِيلِ وَٱلْأَعْنَٰبِ تَتَّخِذُونَ مِنْهُ سَكَرًۭا وَرِزْقًا حَسَنًا ۗ إِنَّ فِى ذَٰلِكَ لَءَايَةًۭ لِّقَوْمٍۢ يَعْقِلُونَ ﴿٦٧﴾
And from the fruits of the palm trees and grapevines you take intoxicant and good provision. Indeed in that is a sign for a people who reason.
The same grapes can give a forbidden intoxicant or a good provision.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Taharah, chapter on impurities, on transformation (inqilab al-'ayn). A substance that fully changes into another takes a new ruling.
- al-Hidaya, Kitab al-Ashriba. Wine that becomes vinegar is lawful, an example of a forbidden substance changing into a lawful one.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.