Rabbi Slifkin seems to think that it is self-evident that passages ascribing “strength” (Hebrew: חזק) to the rakia connote hardness and solidity. But this is not so. “Strength” has many aspects, many of which do not relate at all to hard texture.
Indeed, the Targum on Sefer Iyov, a Tannaitic work —directly on the verse Rabbi Slifkin cites—understands the rakia’s similarity to “a cast metal mirror” in its “strength” not in the aspect of hardness, but in the aspect of the sky’s intensity of clarity or brightness:
תרקע עמיה לשחקיא תקיפין דחזותהון כאספקלריא סנינה
—Not: “the shechakim that are strong, and [in addition] whose appearance is like clear glass;” but:
“the shechakim that are strong/intense, in [the aspect] that their appearance is like a bright looking-glass.”
This Targum alone refutes Rabbi Slifkin’s claim that all Chazal thought this verse in Iyov was saying the rakia is a literally solid substance; and that it was only in the time of the rishonim—who “were ignoring Chazal”!—that the verses were “reinterpreted” otherwise.
And so, unsurprisingly, the talmudic sources bear out the classical commentators, who likewise never say that the terms “congealing,” “strengthening” and “strong as a cast metal mirror” ascribed to the rakia (whether taken to mean the cloud or celestial region) are meant to convey anything to do with “hardness.” Rather, they say these expressions are meant to convey one or more of the following:
(a) the process of the materialization of the rakia on Creation’s second day, from the more ethereal state to the more physical,
(b) the rakia’s stabilization and/or its nature being made permanent,
(d) the strong degree of its transparency,or
(e) the intensity of the rakia’s brightness—as, we have seen, the Targum translates it.
Another illustration of a Torah mindset regarding what Chazal meant by the rakia’s “frosting” and “congealing”: The Mizrachi (Breishis 1:6) brings 4 proofs for the opinion that the Midrash about the "congealing" and "strengthening" of the rakia cannot be referring to the atmosphere, and must be referring to the galgalim. It is noteworthy that not one of the 4 proofs claims the idea of "frosting" or "congealing" cannot apply to the atmosphere! This shows that, to the mind of the Mizrachi, the terms "frosting" and "congealing" are easily consistent with the atmosphere and do not imply hardness or solidity.
In particular, note this proof of his that does involve texture:
וכן ממה שאמר, "לחים היו ביום ראשון ובשני קרשו", לא יתכן זה רק על הגלגלים, לא על הרקיע האויריי, כי לדברי הכל לא נברא ביום ראשון.
The concept "לחים היו ביום ראשון ובשני קרשו" is "only possible as referring to the galaglim," not the atmosphere--why? Not because the concepts fit the galgalim but not the atmosphere, but only because the atmosphere was first created on day 2, so it couldn't have been what was "לח" on day one!
It did not occur to the Mizrachi that these terms indicated that the object spoken of was considered a hard solid! Why? Because whether applied to the atmosphere or the galgalim, they simply do not connote hardness!
Yes, the rishonim knew their Bible and Talmud—better than did 2nd century Christian monks or do 20th century Christian evangelists the likes of Paul Seeley (the latter who thinks that the idea of the rakia being the atmosphere originated with 18th century conservative scholars).
 Just to provide a few familiar expressions from the Siddur as examples:
כי חיזק בריחי שעריך
ובידך לגדל ולחזק לכל
כי גואל חזק אתה
אז בקול...חזק משמיעים קול
חזק ויאמץ לבך
חלומותי...אם טובים הם--חזקם
 “Strength” denotes many aspects other than hardness. This is true in English usage as well as in scriptural usage. See Addendum.
 The online Jewish Encyclopedia dates the work’s composition to before the fall of Rome in 476 A.C.E.
 Temporarily backtracking from his mantra that the rishonim universally believed the rakia to be a hard, solid sphere, the Rationalist (The Big Picture of the Firmament) writes, “[D]uring the time of the Rishonim, Ptolemaic cosmology had already become widely accepted, and some of the Rishonim reinterpreted Tenach to suit it. It is possible that some of them did not believe the rakia to be a substantive firmament… But all this would mean, is that the Rishonim were ignoring Chazal (as Rambam and Ibn Ezra did on several occasions) and reinterpreting Torah in light of science…”
 The Rambam, taking the rakia of day 2 to be the atmosphere, and the Ramban (See Addendum on previous post) understanding it to be the non-earthly-physical star region of the sky, both quote the very Midrash describing the rakia as having been "frosted/solidified/congealed" and explain that Chazal meant by these terms the transformation of more ethereal substance into more physical, although still ethereal substance.
 The Ibn Ezra (בראשית שיטה אחרת - פירוש פרק א), identifying the rakia of Day Two as the atmosphere, citing the Midrash’s comparison of the rakia to a “stretched out cloth tent:
והנה הרקיע הוא זה האויר ועוף תעופף עליו. ועליו כתוב וימתחם כאהל לשבת האדם תחתיו (ישע' מ, כב) כמשפט האהלים. ונקרא שמים בעבור היותו עליון על כל הארץ
The Ralbag on Breishis explains that, in saying the rakia became “frosted/solidified/congealed," Chazal meant that Hashem endowed it with “עמידה וקיום”—permanence.
The Radak (Sefer Shorashim, under יצק) offers this as his first explanation of the Iyov verse as well:
ויתכן להיות בצקת עפר למוצק [איוב לח, לח], ענינו חזק כלומר בהתוך העפר להיותו חזק וקבוע. וכן אמר אחריו ורגבים ידבקו. וכן לבו יצוק כמו אבן (איוב מא, טו) ויצוק כפלח תחתית (שם), כלומר חזק קבוע וכן דבר בליעל יצוק בו (תהלים מא, ט) וכן והיית מוצק (איוב יא, טו) חזק, וכמוהם חזקים כראי מוצק (שם לז, יח) כלומר חזק, ואף על פי שבזה יש לנו טעם טוב מזה שיהיה מהענין הראשון, וכן פירושו אומר על השמים כי הם חזקים וברורים ומזהירים כמראה המותך כי כן עושים מראות מהנחשת המותך יפה שהוא זך ובהיר ומזהיר כמו "ויעש את הכיור נחשת ואת כנו נחשת במראת הצבאות (שמות לח, ח). ויש לפרש כראי שם שלא יהיה מענין במראות הצובאות אבל יהיה ראי מראה ממש ופירושו בהירים ומזהירים כמראה הדבר המותך שהוא מזהיר בשעת התכתו אבל לאחר ימים הוא מעלה חלודה אבל השמים הם בהירים בכל יום כבהירת הדבר המוצק בשעת יציקתו
 Ralbag directly on the tarkia verse in Iyov explains the “strength” of the shechakim to be referring to the non-fragmentizing of the storm clouds, which prevents the sky’s light from shining through:
רלב"ג על איוב סוף פרק לז
האם תרקיע עמו השמים שהם חזקים בתכלית החזוק עד שלא יתפרד דבוקם בשום פנים והם ספירים כמו הראי הנתך ר"ל הזכוכית שיעבור האור בו...שבקצת העתים יהיה הענן חזק עד שלא יראה האור אשר הוא בהיר וזך בשחקים....
 Radak, directly on the Iyov tarkia passage (see note above) and Ralbag.
 Ralbag (ibid.)
 I thank Dr. Jonanthan Ostroff for showing me this Mizrachi.
 “Standard Hebrew lexica and a number of modern biblical scholars have defined the raqia (רקיע "firmament") of Gen 1:6-8 as a solid dome over the earth. Conservative scholars from Calvin on down to the present, however, have defined it as an atmospheric expanse” (The Firmament and the Water Above, The Westminster Theological Journal 53  p. 227). It is a pity and a shame that Rabbi Slifkin relies upon the limited scholarship of an evangelical Christian unfamiliar with classical Jewish sources, as a basis for delegitimizing the mesorah that the Creation process was a meta-natural, 6-day event.
IN ENGLISH, TOO, “STRENGTH” HAS MANY OTHER MEANINGS BESIDES “HARD”
1. having, showing, or able to exert great bodily or muscular power; physically vigorous or robust: a strong boy.
2. accompanied or delivered by great physical, mechanical, etc., power or force: a strong handshake; With one strong blow the machine stamped out a fender.
3. mentally powerful or vigorous: He may be old, but his mind is still strong.
4. especially able, competent, or powerful in a specific field or respect: She's very strong in mathematics. He's weak at bat, but he's a strong fielder.
5. of great moral power, firmness, or courage: strong under temptation.
6. powerful in influence, authority, resources, or means of prevailing or succeeding: a strong nation.
7. aggressive; willful: a strong personality.
8. of great force, effectiveness, potency, or cogency; compelling: strong reasons; strong arguments.
9. clear and firm; loud: He has a strong voice.
10. solid or stable; healthy; thriving: The banker predicted a strong economy.
11. well-supplied or rich in something specific: a strong hand in trumps.
12. having powerful means to resist attack, assault, or aggression: a strong fortress; a strong defense.
13. able to resist strain, force, wear, etc.: strong walls; strong cloth.
14. decisively unyielding; firm or uncompromising: She has strong views about the United Nations. He has a strong sense of duty.
15. fervent; zealous; thoroughgoing: He's a strong Democrat.
16. strenuous or energetic; vigorous: strong efforts.
17. moving or acting with force or vigor: strong winds.
18. distinct or marked; vivid, as impressions, resemblance or contrast: He bears a strong resemblance to his grandfather.
19. intense, as light or color.
20. having a large proportion of the effective or essential properties or ingredients; concentrated: strong tea.
21. (of a beverage or food) containing much alcohol: strong drink; The fruitcake was too strong.
22. having a high degree of flavor or odor: strong cheese; strong perfume.
23. having an unpleasant or offensive flavor or odor, especially in the process of decay: strong butter.
24. of a designated number: Marines 20,000 strong.
25. Commerce . characterized by steady or advancing prices: The market resumed its strong pace after yesterday's setback.
26. Grammar .
a. (of Germanic verbs) having vowel change in the root in inflected forms, as the English verbs sing, sang, sung; ride, rode, ridden.
b. (of Germanic nouns and adjectives) inflected with endings that are generally distinctive of case, number, and gender, as German alter Mann “old man.”
c. belonging to the morphophonemically less regular of two inflectional subtypes.
27. (of a word or syllable) stressed.
28. Optics . having great magnifying or refractive power: a strong microscope.
30. come on strong, Slang . to behave in an aggressive, ardent, or flamboyant manner: When you're interviewed for the job, don't come on too strong.
World English Dictionary
strong (strɒŋ) [Click for IPA pronunciation guide]
— adj , stronger , strongest
1. involving or possessing physical or mental strength
2. solid or robust in construction; not easily broken or injured
3. having a resolute will or morally firm and incorruptible character
4. intense in quality; not faint or feeble: a strong voice ; a strong smell
5. easily defensible; incontestable or formidable
6. concentrated; not weak or diluted
7. a. ( postpositive ) containing or having a specified number: a navy 40 000 strong
b. ( in combination ): a 40 000-strong navy
8. having an unpleasantly powerful taste or smell
9. having an extreme or drastic effect: strong discipline
10. emphatic or immoderate: strong language
11. convincing, effective, or cogent
12. (of a colour) having a high degree of saturation or purity; being less saturated than a vivid colour but more so than a moderate colour; produced by a concentrated quantity of colouring agent
a. denoting or belonging to a class of verbs, in certain languages including the Germanic languages, whose conjugation shows vowel gradation, as sing, sang, sung
b. Compare weak belonging to any part-of-speech class, in any of various languages, whose inflections follow the less regular of two possible patterns
14. (of a wind, current, etc) moving fast
15. (of a syllable) accented or stressed
16. (of an industry, market, currency, securities, etc) firm in price or characterized by firm or increasing prices
17. (of certain acids and bases) producing high concentrations of hydrogen or hydroxide ions in aqueous solution
18. ( Irish ) prosperous; well-to-do (esp in the phrase a strong farmer )
19. have a strong stomach not to be prone to nausea
20. informal in a strong way; effectively: going strong
21. come on strong, to make a forceful or exaggerated impression
Word Origin & History
O.E. strang "physically powerful, powerful in effect, forceful, severe," from P.Gmc. *strangaz (cf. O.N. strangr "strong," Du. streng "strict, rigorous," O.H.G. strang "strong, bold, hard," Ger. streng "strict, rigorous"). Originally compared strenger, strengest (cf. old/elder/eldest ). Grammatical sense, of noun and verb inflections, is first attested 1841, translating Ger. stark, used in a grammatical sense by J. Grimm (the terms strong and weak better fit Ger. inflections). Strong suit (1865) is from card-playing. Strong man “man of great strength" (especially one who displays it professionally) is recorded from 1699; meaning "dominating man in a political organization" is from 1859.