Hieroglyphic writing appears to be a pictographic system, and many think it is similar to Chinese. But while the Chinese use 70,000 or more ideographs, there were only about 800 unique hieroglyphic symbols (with recognizable variations) in use during any period*. That seems too many for an alphabet, but it is much too few for a conceptual script such as Chinese. It is also too many for a convenient syllabary†.
* Hieroglyphs were in use for some 3,000 years. Numerous signs came into use, and others fell out of use, leading to a total corpus of some 5,000 signs used over the centuries.
† The Amharic language of Ethiopia is written with signs that represent syllables, and by my count, uses 282 symbols. The Japanese kana, used for phonetic spelling, number 72 (Japanese also use several thousand kanji, derived from Chinese). Languages such as English that are written with purely alphabetic scripts tend to develop larger numbers of spoken syllables, and thus an English syllabary might need 5,000-10,000 symbols.
The complexities of hieroglyphic script, and the hieratic script that was derived from it, were teased out by a number of workers over many years, beginning with the aforementioned Thomas Young, but the key breakthrough by Champollion was realizing that the script is primarily phonetic in a way similar to Semitic scripts, and that the larger-than-expected number of symbols (linguists call them glyphs) resulted from several factors:
- Like Semitic scripts such as Hebrew, nearly all the phonetic symbols denote consonants.
- Hebrew and many other Semitic scripts use "points" below or within a symbol to indicate the vowel sound that follows the consonant, or an aspiration. Hieroglyphs used a more complex system of determinants, within or above or below a symbol, for a similar purpose.
- More than half the symbols used are indeed ideographs, and denoted whole words. The English ampersand or the +, both used to mean "and", are examples, and we use ¢ instead of writing "cents", £ for "pound Sterling", and ∞ for "infinity".
- Other kinds of determinants were used, in ways I don't understand, but some may have had grammatical meaning such as the "particles" or postpositions in Japanese.
J-F Champollion was a difficult man. Without the protection of his older brother Jacques-Joseph, and other influential men he managed to befriend, he'd have become a forgotten footnote of history. Their father was a book seller, and both boys were bookish. Both became librarians. The Napoleonic expedition to Egypt, when J-F was 11 years old, resulted in an "Egyptian frenzy" in French society. Family friendships with some of the scientists who'd been on that expedition resulted in opportunities to see materials returned to French museums and academic institutions, and ignited this frenzy in young J-F, called Champollion jeune ("the younger") in writings of his time, as his brother became known as Champollion-Figeac, after their birth city.
His labors to decipher the Egyptian scripts were based on his knowledge of Coptic, taught him by a priest who'd been sheltered in the Champollion house during the Republic, when priests were persecuted. This was both a help and a hindrance, because while Coptic has many cognates with ancient Egyptian, there are many false cognates and other words that are simply different. Words in two related languages have a cognate relationship if they are spelled and/or pronounced similarly and have very similar meanings, such as "wine" in English and its Italian cognate "vino"; in Old English, AKA Anglo-Saxon, the "W" was pronounced as a "V". False cognates are words that seem similar but aren't. Thus in French, "assister" does not mean to assist or help, but to "attend", such as going to a symposium. Because I know French, I can sometimes read a sentence or two of the related language Spanish, but usually find Spanish mostly unreadable, and I cannot understand hardly any spoken Spanish.
We must also remember that the Egyptian spoken language of 3000 BCE was probably quite different from that of 196 BCE, the time that the Rosetta Stone was made. This, and the nearly universal opinion in the early 1800s that Hieroglyphic was almost entirely conceptual (ideographic like Chinese), led Champollion down numerous blind alleys during the two decades (1802-1822) prior to his breakthrough.
The publication of a system that allowed a scholar to read most hieroglyphic and demotic inscriptions led to a burst of activity, and fame for Champollion. He obtained sponsorship for a joint French-Italian expedition to Egypt in 1828-9, and was made the first Professor of Egyptology upon his return. Sadly, the sickly J-F had not long to live. He died early in 1832. The enemies he had made in his life continued their attacks after his death, and delayed acceptance of his system. But once scholars began to test it and found it useful, much work went on to expand it, and to correct a few matters, leading to the ability to read nearly all (but not quite all) hieroglyphic papyri and inscriptions.
An interesting result of Champollion's work, and an area where even he had the wisdom to tread lightly, was his discovery that the early Egyptian dynasties dated to several hundred years prior to the accepted date of the Biblical Flood. Modern Biblical chronologies still place the Flood in either 2268 BCE, or less frequently a range of dates reaching to about 2500 BCE. The 280 kings, in 31 dynasties of Egypt, prior to 332 BCE when Alexander took over, reach back to about 3050 BCE when you add up the years that each reigned. Also, a number of earlier documents discuss "legendary" events described as 2000 to 3000 years earlier than the First Dynasty, indicating times that predated the accepted date of Adam's creation in about 4000 BCE by many centuries. In the 1820s, to claim such things might be fact was to put oneself in mortal danger.