Such a fall in the value of labor-power [from 10 work-hours to 9 work-hours] implies, however, that the same necessaries of life which were formerly produced in ten hours can now be produced in nine hours. […] and this cannot be done, except by an alteration in his tools or in his mode of working, or in both. Hence, the conditions of production, i.e., his mode of production, and the labour process itself, must be revolutionised. By increase in the productiveness of labour, we mean, generally, an alteration in the labour process, of such a kind as to shorten the labour time socially necessary for the production of a commodity, and to endow a given quantity of labour with the power of producing a greater quantity of use value. Hitherto in treating of surplus value, arising from a simple prolongation of the working day, we have assumed the mode of production to be given and invariable. But when surplus value has to be produced by the conversion of necessary labour into surplus labour, it by no means suffices for capital to take over the labour process in the form under which it has been historically handed down, and then simply to prolong the duration of that process. The technical and social conditions of the process, and consequently the very mode of production must be revolutionised, before the productiveness of labour can be increased. By that means alone can the value of labour power be made to sink, and the portion of the working day necessary for the reproduction of that value, be shortened. Marx, Capital, Volume 1, Marx Engels Collected Works Volume 35 (International Publishers), p.319-320

The exceptionally productive labour operates as intensified labour; it creates in equal periods of time greater values than average social labour of the same kind. […] the capitalist who applies the improved method of production, appropriates to surplus labour a greater portion of the working day, than other capitalists of the same trade. […] On the other hand, however, this extra surplus value vanishes, so soon as the new method of production has become general, and has consequently caused the difference between the individual value of the cheapened commodity and its social value to vanish. The value of commodities is in inverse ratio to the productiveness of labour. And so, too, is the value of labour power, because it depends on the values of commodities. Relative surplus value is, on the contrary, directly proportional to that productiveness. It rises with rising and falls with falling productiveness. […] if, in consequence of increased productiveness, the value of the necessaries of life fall, and the value of a day's labour power be thereby reduced from five shillings to three, the surplus value increases from one shilling to three. […] Hence there is immanent in capital an inclination and constant tendency, to heighten the productiveness of labour, in order to cheapen commodities, and by such cheapening to cheapen the labourer himself. The value of a commodity is, in itself, of no interest to the capitalist. What alone interests him, is the surplus value that dwells in it, and is realisable by sale. […] Now, since relative surplus value increases in direct proportion to the development of the productiveness of labour, while, on the other hand, the value of commodities diminishes in the same proportion; since one and the same process cheapens commodities, and augments the surplus value contained in them; we have here the solution of the riddle: why does the capitalist, whose sole concern is the production of exchange value, continually strive to depress the exchange value of commodities? […] The shortening of the working day is, therefore, by no means what is aimed at, in capitalist production, when labour is economised by increasing its productiveness. It is only the shortening of the labour time, necessary for the production of a definite quantity of commodities, that is aimed at. he fact that the workman, when the productiveness of his labour has been increased, produces, say 10 times as many commodities as before, and thus spends one-tenth as much labour time on each, by no means prevents him from continuing to work 12 hours as before, nor from producing in those 12 hours 1,200 articles instead of 120. Marx, Capital, Volume 1, Marx Engels Collected Works Volume 35 (International Publishers), p.323-326

In order to effect a fall in the value of labour power, the increase in the productiveness of labour must seize upon those branches of industry, whose products determine the value of labour power, and consequently either belong to the class of customary means of subsistence, or are capable of supplying the place of those means. But the value of a commodity is determined, not only by the quantity of labour which the labourer directly bestows upon that commodity, but also by the labour contained in the means of production. For instance, the value of a pair of boots depends, not only on the cobbler's labour, but also on the value of the leather, wax, thread, etc. Hence, a fall in the value of labour power is also brought about by an increase in the productiveness of labour, and by a corresponding cheapening of commodities in those industries which supply the instruments of labour and the raw material, that form the material elements of the constant capital required for producing the necessaries of life. But an increase in the productiveness of labour in those branches of industry which supply neither the necessaries of life, nor the means of production for such necessaries, leaves the value of labour power undisturbed. Marx, Capital, Volume 1, Marx Engels Collected Works Volume 35 (International Publishers), p.320

The surplus value produced by prolongation of the working day, I call absolute surplus value. On the other hand, the surplus value arising from the curtailment of the necessary labour time, and from the corresponding alteration in the respective lengths of the two components of the working day, I call relative surplus value. Marx, Capital, Volume 1, Marx Engels Collected Works Volume 35 (International Publishers), p.320

The object of all development of the productiveness of labour, within the limits of capitalist production, is to shorten that part of the working day, during which the workman must labour for his own benefit, and by that very shortening, to lengthen the other part of the day, during which he is at liberty to work gratis for the capitalist. Marx, Capital, Volume 1, Marx Engels Collected Works Volume 35 (International Publishers), p.326

[…] let some one capitalist contrive to double the productiveness of labour, and to produce in the working day of 12 hours, 24 instead of 12 such articles. […] If therefore, the capitalist who applies the new method, sells his commodity at its social value of one shilling, he sells it for threepence above its individual value, and thus realises an extra surplus value of threepence. […] This augmentation of surplus value is pocketed by him, whether his commodities belong or not to the class of necessary means of subsistence that participate in determining the general value of labour power. Hence, independently of this latter circumstance, there is a motive for each individual capitalist to cheapen his commodities, by increasing the productiveness of labour. Marx, Capital, Volume 1, Marx Engels Collected Works Volume 35 (International Publishers), p.322-323