Черепок стал хрупким, оттого что обжиг закончился слишком рано.

After стать in the sense of to become, Russian normally puts the noun or adjective describing the new state into the instrumental:

• стать хрупким = to become brittle
• стать врачом = to become a doctor

So Черепок стал хрупким literally follows the very common pattern X became Y.

Using хрупкий here would sound wrong in standard Russian.

Here стал is the past tense of стать and means became.

So:

• черепок стал хрупким = the shard became brittle

This is not related to стоять (to stand), even though English speakers sometimes confuse them at first.

Also, стал emphasizes a change of state. If you said был хрупким, that would mean was brittle, not became brittle.

Because the sentence describes a change.

• стал хрупким = became brittle
• был хрупким = was brittle

So the Russian is focusing on the moment or result of the change: the shard was not in that state before, and then it ended up brittle.

Оттого что means because, since, or as a result of the fact that.

In many contexts, it is very close to потому что. However:

• потому что is the most neutral and common everyday way to say because
• оттого что can sound a bit more literary, explanatory, or formal

In this sentence, оттого что connects the result and its cause:

• the shard became brittle
• because the firing ended too early

So yes, you could usually replace it with потому что without changing the basic meaning.

Because оттого что introduces a subordinate clause.

Main clause:

• Черепок стал хрупким

Subordinate clause:

• оттого что обжиг закончился слишком рано

Russian normally separates this kind of clause with a comma, so the comma is required here.

Yes. In this sentence, оттого is written as one word because it is part of the conjunction оттого что.

• оттого что = because

This is different from от того as two separate words, where того is the pronoun that in a case form.

Compare:

• Он дрожал оттого, что замёрз. = He was shivering because he got cold.
• От того дома ничего не осталось. = Nothing remained of that house.

So in your sentence, it is the conjunction, so оттого is one word.

Обжиг is a noun meaning firing in the ceramics sense: heating clay in a kiln.

So:

• обжиг закончился = the firing ended

It is a masculine singular noun, which matters because the past-tense verb agrees with it.

Because закончиться means to end, to come to an end, or to be over.

Compare:

• закончить что-то = to finish something
  
  • Мастер закончил обжиг. = The craftsman finished the firing.
• закончиться = to end, to be finished
  
  • Обжиг закончился. = The firing ended.

So here the firing itself is the subject, and Russian uses the intransitive/reflexive form закончиться.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject here is обжиг, which is:

• singular
• masculine

So the verb must also be masculine singular:

• обжиг закончился

If the subject were feminine, you would get закончилась.
If it were plural, you would get закончились.

Both verbs present events as completed facts.

• стал shows that the shard ended up in a new state
• закончился shows that the firing came to an end

That fits the meaning of the sentence: it is talking about a result and its cause.

If you used imperfective forms, the meaning would shift toward process, repetition, or background description.

Слишком рано means too early.

• слишком = too, excessively
• рано = early

In the sentence, it modifies закончился:

• обжиг закончился слишком рано = the firing ended too early

Yes, the word order can change somewhat in Russian:

• обжиг слишком рано закончился
• слишком рано обжиг закончился

But the original order is natural and neutral.

Russian has no articles, so черепок can mean either a shard or the shard, depending on context.

You figure it out from the wider situation:

• if the shard has already been mentioned, English will often use the
• if it is being introduced for the first time, English will often use a

So the Russian sentence itself does not mark that distinction directly.

The stress is:

черепо́к стал хру́пким, оттого́ что обжи́г зако́нчился сли́шком ра́но

A few especially useful ones to remember are:

• черепо́к
• хру́пкий
• обжи́г
• зако́нчился
• сли́шком

Stress is important in Russian, so it is worth learning these together with the words.