Does English allow a zero copula in subordinate clauses?

In a casual search of the web, I found a few indications English does not allow zero copulas (https://linguistics.stackexchange.com/a/1468). However, I frequently see sentences with subordinate clauses that juxtapose a noun phrase with an adverb phrase, such as the following from a contemporary sci-fi author:

Kai stares up at me from the grave, his eyes hard as obsidian.

If this clause were made into a sentence, it would take an obligatory verb:

*His eyes hard as obsidian.
His eyes are hard as obsidian.

The original clause seems to contain an implicit "to be." Is it an example of a zero copula? If not, what is this construction called?

Solution 1:

It depends on your definition of a clause. The traditional definition is "a finite verb and all its dependencies"; then your phrase is not a clause.

The construction is called an absolute construction: a noun and an attribute together forming an adverbial constituent.

Solution 2:

There are any number of situations in which be can be deleted (or, alternatively, prevented from appearing) in subordinate -- and even main -- clauses.

A very simple example is Whiz-Deletion, which applies to restrictive relative clauses, and deletes (as I will call it) the subject relative pronoun (the "Wh-" part) plus whatever inflected form of be it agrees with (the "-iz" part), relating, for instance,

• The man who is standing on the corner is winking at me
  and
• The man standing on the corner is winking at me,
  thereby also relating most post-nominal phrasal modifiers to equivalent relative clauses.

Another is to be-Deletion, which applies to infinitive complement clauses, and is governed by the predicate, like the infinitive complementizer it modifies.

Verbs that allow to be-Deletion include declare, consider, and feel.

• He declared/considered/felt Max (to be) responsible for the accident. [optional to be]

Verbs that don't allow it include state, know, hear, and intend.

• He stated/knew/heard/intended Max to be responsible for the accident.
• **He stated/knew/heard/intended Max responsible for the accident.* [obligatory to be]

A third rule, Conversational Deletion, applies only in main clauses, at the very beginning of a sentence. It deletes any predictable (i.e, meaningless, dummy, auxiliary, grammar particle, etc.) element, producing such normal utterances as

• Too bad about old Charlie. [from It is too bad]
• No need to get upset about it. [from There is no need]

Note that none of these delete copulas alone, but only as part of some complex, usually with a subject and/or other function word being deleted as well. There's no reason to delete a copula until it's not needed any more.