The commands to build an earthen or stone altar in Shemot 20 refer to altars which were distinct from the wood altar of the Tabernacle in Shemot 27. Commentators divide regarding the identification of the altars in Shemot 20:
The commandments to build earthen and stone altars refer to the building of a special altar for each of two unique covenantal ceremonies, one of which took place at the foot of Mt Sinai, and the other on Mt. Eival.
"מִזְבַּח אֲדָמָה" – Ibn Ezra identifies the earthen altar with the altar Moshe built at Mt. Sinai in Shemot 24.
9 This altar is referred to by this name because it was actually made from earth.
10 "מִזְבַּח אֲבָנִים" – Ibn Ezra identifies this stone altar with the only altar which the Torah explicitly states was to be made of stone, the altar to be built on Mt. Eival described in Devarim 27.
The Altar in the Tabernacle – The altar in the Tabernacle was made of copper plated wood, rather than earth or stone, and is thus unconnected to the altars discussed in Shemot 20.
One time command – According to Ibn Ezra, the commands of Shemot 20 were one time commands to be fulfilled on a single occasion. They were not meant for all generations, but for Moshe (and Yehoshua) alone.
"נְבוּב לֻחֹת" – Ibn Ezra understands this phrase to mean that the Tabernacle's altar was hollow, and filled with earth. He emphasizes, though, that this is not sufficient reason to call such an altar an "earthen altar", and thus the altar of the Tabernacle cannot be the subject of the command in Shemot 20.
Would the wood not burn? – As the altar was filled with earth, there was no concern of it burning.
No steps for modesty – Since the altars in Shemot 20 were not part of the Tabernacle and the regular priestly rites, it was necessary to include a warning not to use steps, so as to prevent immodest exposure if their service was performed in robes (without pants).
"בְּכָל הַמָּקוֹם אֲשֶׁר אַזְכִּיר אֶת שְׁמִי" – Ibn Ezra explains that this refers to all the places in which God chooses to have his glory reside, such as Shiloh and Nov. If one visits (and presumably sacrifices at) such places, then Hashem will visit and bless you in return. According to Ibn Ezra, though, it is not clear why this is mentioned in Shemot 20, as it is unrelated to the altars being discussed there.
"...וְאִם מִזְבַּח" – Ibn Ezra understands the word "וְאִם" to mean "and if" and explains that Hashem is telling Moshe that he presently needs to build an earthen altar, and if he merits to enter the Land of Israel, he will then build a stone one.
11 Location of the command – Ibn Ezra suggests that the command to build the earthen altar is juxtaposed with the commandment not to make other gods, as this altar was to be built for the Sinai covenant of Chapter 24, in which the nation accepted Hashem as their sole God. It is not clear, though, why Hashem also relayed the command to the build the stone altar here, given that the covenantal ceremony at Mt. Eival was to take place only much later.
12 In addition, when the full ceremony is commanded in Devarim, the instruction to build a stone altar is repeated, making the command in Shemot 20 seemingly redundant.
Chronology of Chapter 24 – Ibn Ezra must maintain that the events of Chapter 24 occur in their chronological place, and not, as others suggest, in Chapter 19 and before the giving of the commandments in Chapter 20.
13 The verses which speak of earth and stone altars refer to private altars (במות) used by non-priests outside the Tabernacle and Temple for either sacrificial worship or slaughtering of meat.
"מִזְבַּח אֲדָמָה" – According to Yefet the Karaite and R. Hoffmann, the command regarding the earthen altar refers to the building of altars for private sacrificial worship outside the sanctuary.
14 Both limit the locations of such altars to places sanctified by God through revelation or the like ("בְּכָל הַמָּקוֹם אֲשֶׁר אַזְכִּיר אֶת שְׁמִי").
15 "מִזְבַּח אֲבָנִים" – The commentators differ regarding the function of these altars and how they relate to the earthen ones of the previous verse:
- Bamot for private sacrificial worship – Yefet asserts that the altars for private sacrifice could be made from either earth or stone and that this verse, too, is a directive allowing such temporary altars.
- Slaughtering בשר תאווה – Two other Karaites, Nahawandi and Hadassi, maintain that this verse is referring to building stone altars for the slaughtering of non-sacrificial meat. They point to Shemuel I 14:32-35 as evidence, where Shaul commands his soldiers to slaughter their sheep and cattle on a large stone, instead of pouring the blood on the ground.16
- Permanent altars – R. D"Z Hoffman suggests that this command refers to the permanent and centralized altars of Mt. Eival, Shiloh and the Beit HaMikdash. According to R. Hoffmann, the earthen altars were allowed during periods of insecurity and unrest but during eras of peace, only one centralized, altar was to be used. As this was meant to be a permanent structure, it was made of stone.17
When and why were private altars allowed? – According to Yefet, sacrificing Olot or shelamim outside the Tabernacle was only allowed during periods in which the Aron was separated from the Tabernacle. This is perhaps related to the idea that in such periods there is less holiness in the sanctuary. R. D"Z Hoffmann asserts, in contrast, that temporary altars were allowed during periods when lack of peace and security made it difficult for a person to to get to one centralized location.
18 To highlight the temporary aspect of these altars, earth was chosen as the building material since it represents a lack of permanence.
The Altar in the Tabernacle – The command to build the altar of the Tabernacle is unconnected to the commands in Shemot 20.
19 "בְּכָל הַמָּקוֹם אֲשֶׁר אַזְכִּיר אֶת שְׁמִי" – According to Yefet and R. Hoffmann, this phrase limits the places in which a private individual can build a temporary altar for sacrificial worship to sites in which God's name is somehow revealed.
20 "וְאִם מִזְבַּח אֲבָנִים" – According to Yefet, the verse is providing two equally valid options for the material from which one might build private altars. One might build from earth, or if one wants, from stone.
21 R. Hoffmann, in contrast, understands the word "אם" to mean "when."
22 "נְבוּב לֻחֹת" – R. Hoffmann follows the Midrash in explaining that the altar of the Tabernacle was hollow and filled with earth. It is not clear how the Karaite commentators explain the phrase.
Would the wood not burn? – According to R. Hoffmann, the altar was filled with earth, so there was no concern of burning.
No steps for modesty – According to this approach, the verse is aimed at individual Israelites who would not necessarily be wearing pants.
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