A.S. Yahuda, Language of the Pentateuch in its Relation to Egyptian [1933], Vol. 1, [1933], 69 The two most frequent expressions used to characterize the stubbornness and obstinacy of Pharaoh are: ויכבד לב פרעה 'and Pharaoh's heart was heavy' Ex. 9,7. also 7, 14. and in the causative ויכבד פרעה את־לבו 8, 27. 9, 34 etc. or ויחזק לב פרעה 'and the heart of Pharaoh became strong' 7, 13, 22. 8, 15. 9, 35 and in the causative ויהוק יהוה את לב פרעה 'and God made the heart of Pharaoh strong' 9, 12. 10, 20, 27, also 3, 21 etc. In both cases we have an exactly literal coincidence with the Egyptian, for כבד = Egyptian dnś 'to be heavy (in weight)' which in conjunction with ib = heart likewise means 'to be of fixed heart, fixed mind, to be resolved', e.g. Siut, i. 81 'I, the ruler of love, heavy of heart (dnś ib), I conceal the thought of the body (hrp śhr h.t)', i.e. obstinately, with firm determination, I do not reveal my feelings. In other cases dnś = 'to be heavy' is used without ib in the same sense; thus Bauer, i. 209 (= Vogels, p. 160) gives dnś śhr h.t) = 'heavy in regard to the thoughts of the body' meaning 'to be firm, resolved', in contrast to isy-ib 'light in regard to the heard' i.e. wanton, frivolous, vacillating. 'Heavy of heart', thus means to be stubborn, insistent; and 'to make the heart heavy' means 'to show forbearance, patience, endurance'. That is precisely what the passages cited convey: Pharaoh did not despair, he was patient, steadfast, resolved, and obstinate.
Vertaling Bijbel, Kanttekeningen SV, [], Alzo werd Farao's hart verstokt, dat hij de kinderen Israels niet trekken liet, gelijk als de HEERE gesproken had [30]door Mozes. 30. Hebreeuws, door de hand van Mozes; dat is, door Mozes' dienst, als zijnde het instrument hetwelk God de Heere gebruikt heeft. Zie deze wijze van spreken, hfdst.35 vs.29; Lev.8:36; 2 Sam.11:14; 2 Kon.17:13; Hag.1:1; Mal.1:1, en elders meer.