Some have criticized the Book of Mormon for claiming to be written in a language it calls "Reformed Egyptian" onto gold plates. Early on, scholars claimed that the notion of such a concept was preposterous. Hebrews would never write sacred writings and scripture in a "pagan" language, and there is no evidence that Ancient Israelites wrote on metal plates.
Morm. 9: 32 And now, behold, we have written this record according to our knowledge, in the characters which are called among us the reformed Egyptian, being handed down and altered by us, according to our manner of speech.
While there is no "smoking gun" proof of the exact reformed Egyptian the Book of Mormon was written in. There exist numerous examples in the archaeological record of many forms of "Reformed Egyptian" developed in 600 B.C. when Lehi left Jerusalem for America. There are also many example of artifacts from the same time period proving that Israel and neighboring countries recorded religious, civic, and economic information on metal.
The earliest form of Egyptian writing is hieroglyphics (Greek for "sacred symbols"). Hieroglyphics comprised nearly 400 picture characters depicting things found in real life and represented the various phonemes in Egyptian speech. While an impressive and ornate way to record language, hieroglyphics was a cumbersome way to record language on a day-to-day basis. Therefore, a new script called hieratic (Greek for "sacred") was developed by Egyptian priests for use on papyrus. Hieratic was a hieroglyphic shorthand which only used a simplified portion of the original glyph. Then around 700 B.C., the Egyptians developed an even more cursive and simplified script that we call demotic (Greek for "popular"), which bore little resemblance to the hieroglyphs. Both hieratic and demotic were in use in Lehi's time and can properly be termed "reformed Egyptian."
There are other forms of "reformed Egyptian" which were developed at the same time as Demotic such as Meroitic which was used for over a thousand years in the Nubian Kingdom of the upper Nile region. Today, these scripts are all considered Egyptian; but at the time to those who first started using them, these new scripts would have been considered "reformed."
Upon first impression, the grammar and style of the Book of Mormon is very similar to the King James Version (KJV) of the Bible. The way in which sentences are formed and thoughts are conveyed are Hebrew and not Western European. In fact, the Book of Mormon is more Hebrew-like than the Bible because of its many “Hebraisms.” Critics contend that if the text had been originally written in Egyptian and then translated into English by Joseph Smith, that it shouldn't be nearly as Hebrew-like in grammar, syntax, and sentence structure; or, shouldn't follow the KJV so closely. If written in Egyptian, the Book of Mormon should be loaded with “Egyptisms” and not “Hebraisms.”
However, this criticism forgets an important detail. Moroni says they used reformed Egyptian characters on the Gold Plates, but their language and learning was Hebrew.
1 Ne. 1: 2 Yea, I make a record in the language of my father, which consists of the learning of the Jews and the language of the Egyptians.
Therefore, it is most likely that the Book of Mormon was written in Hebrew using an Egyptian alphabet. While strange sounding, this idea of writing a language in different scripts is very common place. For example, Urdu and Hindi are 2 of several major languages in India. They actually are the same language but are only written with different scripts. Urdu and Hindi speakers can understand each other's spoken word nearly 100%. However, while Urdu is written in Perso-Arabic, Hindi is written in Sanskrit.
The significance of these examples is that the invention of writing Hebrew using Reformed Egyptian script is probably not an Ancient Israeli invention but an Egyptian invention. There is ample evidence to support the idea that some educated Egyptians learned and spoke Semitic languages such as Hebrew and wrote them in their native scripts. Lehi would be familiar with the Egyptian writing system through his work in the caravan and trade business.
The Book of Mormon suggests that the Brass Plates (Hebrew Bible) Lehi brought with him to the America's were written in this style and served as a template for the Gold Plates (Book of Mormon).
Mosiah 1: 4 For it were not possible that our father, Lehi, could have remembered all these things, to have taught them to his children, except it were for the help of these plates; for he having been taught in the language of the Egyptians therefore he could read these engravings, and teach them to his children, that thereby they could teach them to their children, and so fulfilling the commandments of God, even down to this present time.
This verse of the Book of Mormon should not be interpreted to suggest that writing Hebrew on metal plates was a common place. The Brass Plates of Laban (wealthier cousin of Lehi) were considered extremely valuable and rare, as was Laban's “precious steel” sword, and was kept in Laban's treasury, even when Zoram, Lehi's servant, indicated to Nephi that Laban had been out with the Elders. If any reading from the scriptures took place it could have been done from everyday parchment or leather scrolls.
The reason the Book of Mormon suggests why the Brass Plates and the Book of Mormon itself were written in Egyptian script and not Paleo-Hebrew is efficiency of space. I am not an expert of ancient languages but just looking at the Rosetta Stone, writing in hieroglyphics and demotic requires many less symbols then writing in Coptic.