'hardens' is correct and usable in written English.
You can use it as an intransitive verb meaning 'to become hard or harder.' For example, "As the paint dries, it hardens."
You roll your mind around enough in that stuff and gossamer becomes shellac, and your worldview hardens to the point of impermeability.
David Cameron will on Wednesday set a deadline to hold an in-out referendum on Britain's membership of the European Union by the end of 2017 as he hardens his position on the issue that has bedevilled Tory leaders for a quarter of a century.
A lot of glassblowing is about timing: how long you heat it, the time you have to shape it before it hardens (about 30 seconds) and when to stop before you potentially destroy it.
A team led by Ellery Frahm from the University of Sheffield has found a way to pinpoint the source of artefacts made of obsidian, a glassy rock formed after volcanic lava hardens, on the spot, in ten seconds.Given the painstaking nature of archaeological work, and consequently its leisurely pace, the technique may sound like overkill.
As the exhibition unfolds the older artist's vision hardens.
The composite fills the crack and then hardens in situ.
As public opinion hardens against membership of the euro, the government is growing uneasy about having decided in principle to take Britain into the single currency.
If public disapproval hardens, critics will be emboldened to speak out.
When it hardens, 20 minutes later, the grancrete structure is twice as strong as traditional concrete, and it is durable, fire-resistant, waterproof and non-toxic more thanon-toxic morefor mosthan the poor's housing.
IT HARDENS anything from saw blades to turbine blades.
