Gandhi, using the top 12 blade for 1000 years, could not harden it. But judging by Saturn, the Horosei are masters of acoc. That is, the swordsman Nandi (who in my opinion is number 3 in strength, after Mihawk and Shanks) is not strong enough for this. Another proof that it is the power of haki and the swordsman in general that allows you to harden the blade and increase its level, and not the duration of wearing.Only the Sword God and the Strongest Swordsman have achieved this.