Just to clarify definitions, phonograms are symbols that are used to represent sounds. Ideograms are symbols used to represents objects or concepts.
The ancient Egyptian hieroglyphic alphabet seems to have included both phonograms and ideograms from the very beginning, somewhere between 3300 and 3200BC. Words are often constructed using phonograms at the beginning of the word, followed by an ideogram (which we call the "determinative" of the word).
The ancient Egyptian phonograms only represented the sounds of consonants (singly or in combination). They didn't write vowels, which is why - in most cases - we actually do not know how they pronounced words. There are quite a number of words which, when written in phonograms, look identical (although the determinative allows us to distinguish between them). It may be that in many, perhaps most, of these cases, the ancient Egyptians used different vowel sounds so that the the words were actually pronounced quite differently.
The ancient Egyptian language was written in three scripts:
- Hieroglyphs - the form that most people are familiar with from
pictures of ancient Egyptian tombs and temples.
- Hieratic - a cursive form of hieroglyphs used for writing on
papyrus or ostraca with reed pens and ink. (When the hieratic symbols
are well-formed I've often found it possible to recognise the
correlation between hieratic and hieroglyphic symbols).
- Demotic - an even more simplified version of hieratic. This came
into use quite late, perhaps towards the end of the Third
Intermediate Period in the early 7th century BC. (I was never really
able to get a handle on this one. To my eye it remains little more
than dots and squiggles!).
All three forms were in use throughout the Hellenistic Period. For example, the Rosetta Stone is inscribed with a decree issued by Ptolemy V in 186BC. The decree is written in three scripts: Ancient Greek, hieroglyphs and demotic. Clearly, people were still speaking the ancient Egyptian language and writing using phonograms in 186BC.
In fact, the last firmly datable hieroglyphic inscription we have found so far comes from the temple Isis at Philae, and was written in 394AD.
Now, as mentioned in the page linked by Mark above, it is true that the ancient Egyptians introduced a significant number of new glyphs during the Hellenistic period. The Greeks also introduced new gods/cults and temples in the 3rd century BC. Perhaps the best known of these is the god Serapis who was (probably) introduced by Ptolemy I (it is also possible that Serapis was an existing god whose prominence was raised by Ptolemy I).
The Ptolemies adopted many of the symbols of Egyptian kingship (Ptolemy I assumed the title "Pharaoh" in 305/4 BC), but introduced the Greek language as the official language of court. One should always be cautious when it comes to trying to understand the motivation of ancient peoples, but it seems likely that the changes in dynasty, with the associated changes in language and religious practices, and the modifications to the hieroglyphic writing system are related.
Sources:
- Allen, James P: Middle Egyptian, Cambridge University Press,
2000
- Betro, Maria Carmela: Hieroglyphics: The Writings of Ancient
Egypt, Abbeville Press, 1996
- Falukner, Raymond O: A Concise Dictionary of Middle Egyptian,
Griffith Institute, 1972
- Gardiner, Alan H.: Egyptian Grammar, Griffith Institute, 1957
