When and why did Asian and Southern people start to be called "чурки"?

Wiktionary gives eight different meanings as well as the etymological origin of the word "чурка": Meanings 1-4 are various small pieces of wood or metal, Meaning 5 is a simpleton or uneducated person, Meaning 6 is "someone whose face features are different from those of the Russians – a darker skin, slant eyes, etc.," Meanings 7-8 are regional dialect meanings, and the etymological origin is "чуръ," an old word for stump (link).

Although the original meaning of "чурка" was a piece of wood, I see in Google that nowadays the most usual meaning is Meaning 6 – a Southern or Asian ethnic origin. Here are a few of typical examples:

Фархад, хотя и чурка, но водитель классный, — стал делиться с Раздолбаем Мартин, нисколько не смущаясь присутствия самого Фархада. — Водитель всегда должен быть чуркой. Русских нельзя брать водителями. (Source)

В твоём возрасте у меня был одноклассник из Дагестана. Несмотря на то, что чурка, он был одним из достойнейших людей. (Source)

Любопытно, что я, как русский воспитанный в СССР, был вхож во все местные общины. Потому что в русской культуре нет снобизма. Да, наша культура грубоватая, мы можем сказать в лицо чурке, что он чурка, но мы сразу же принимаем любого человека за равного и если человек правильный, то общение пошло. (Source)

The word is pretty frequently used nowadays.

Analyzing the Google Books statistics, I found that the word has been being actively used at least since the 1800s (link), when it was used in books only about twice as fewer per unit of text length as it is used in books nowadays.

Opening books of that epoch, I saw use of various meanings of "чурка," including a couple of apparent instances of Meaning 6 (ethnic origin):

Он всегда был смуглъ, сутуловатъ, морщинистъ и грубоватъ, – такъ и остался; словом, ничего не прибавилось. Бѣдный старый Чурка! (From a book of 1866)

Но, по счастiю, оказался он живъ, а только опаленъ какъ чурка, и кровь лилась съ лица и со всей головы его. (From a book of 1873)

So it seems the Russians already called Asians and Southern people чурки in the 1800s, but I have no idea whether, for example, the Russians called the Mongols чурки during the Mongol invasion of Rus in the 1200s.

(Update: As @tum_ pointed out in a comment below, I was terribly wrong. In the example of 1868, the word Чурка does not mean an ethnic origin. The text from which I had taken the sentence turned out to be an old Russian translation of the English text entitled "The Tale of Aunt Margaret’s Trouble," and Чурка is how the Russian interpreter translated the nickname "Stock," which, in turn, was derived from "Stork." In the example of 1873, "опален как чурка" can be understood as "burned like a piece of wood." Opening other books of the 1800s, I was unable to find any example where "чурка" means an ethnic origin. So it is quite possible that the Russians did not yet call Asians чурки in the 1800s.)

My question is this: When and why did the Russians start to call people with darker skin and/or slant eyes чурки, а word for а piece of wood? I am curious what the logical connection was.

I am especially interested because as a Japanese I myself qualify as чурка.