What is the difference between "härten", "verhärten", and "abhärten"?

Question

I am looking for the difference between these three verbs. All dictionaries I’ve looked at suggest “harden”, and the boundaries between them are hard to distinguish. From what I gather:

1. Somebody “härtet” something physical.

   Der Schmied härtet den Stahl.

2. Something “verhärtet” itself.

   Der Stahl verhärtet sich durch die Kälte.

3. “Abhärten” is figurative.

   Der Krieg härtet die Soldaten ab.

Where I am not sure is if the distinction between 1 and 2 is always correct. Can somebody not “verhärten” something else? Also, is “abhärten” a close to translation to “toughen up”? For example, if somebody is complaining about the cold weather, can you tell them “du musst dich abhärten” or “you need to toughen up!”?

Answer 1

You are on a very good track.

Abhärten is mostly used when people are hardened against something by being subjected to it and increase their tolerance:

Die Spartaner schliefen in der Kälte um sich abzuhärten./The Spartans slept in the cold to harden themselves.

Your example regarding toughening oneself is also correct:

du musst dich abhärten/you have to toughen yourself

Härten is mostly used when something is made physically hard:

Die Speerspitze wurde im Feuer gehärtet/the spear tip was hardened in fire

Thetis härtete die Haut ihres Sohnes im Feuer/Thetis hardened her son‘s skin in fire

But Duden also lists intransitive use:

Beton härtet innerhalb weniger Tage/Concrete hardens within a few days time

which (I agree with comments) is unusual and use in the sense of abhärten:

sich durch Sport härten/toughen oneself through physical exercise

Verhärten is mostly used when something becomes rigid and inflexible, often with a negative connotation. Duden has good examples:

1. Die Not hat ihr Herz verhärtet/Hardship hardened her heart

2. Die Geschwulst hat sich verhärtet/The tumour hardened

3. Verhärtete Muskeln/tense muscles

4. Die Fronten haben sich verhärtet/Positions became more entrenched

But neutral connotations are also possible. Again, from Duden:

5. das Feuer verhärtet den Ton

If I correctly understand your question, you are mainly unsure about whether verhärten can be used transitively. As you can see from 1. and 5., it can.

Answer 2

I pretty much liked the other answer. It's correct! But here is another explanation that may be of help.

You may think about directions like who/what is hardening who/what or somethings from outside-in our inside-out or active versus passiv.

Verhärten This is about something that may happen naturally. Hot and soft wax will get hard again. Ice cube will crystalize from liquid state to ice inside out. It is the same meaning as aushärten whereas for instance concrete hardens. This all happens in a natural way. You may think of inside-out. In medical terms this is usually a negative meaning since it often indicates deseases. For instance a hard belly is an indicator that you should consult a doctor. One comment above named a discussion or a war that is stuck. This is also a natural development if both parties do not come to an understanding or peace they both will get stuck in a deadlock (with hard fronts) until there is a mediator/negotiator resolves the issue.

Härten This is the opposite to verhärten. In order to harden something you need to apply some force onto it. Otherwise it will never ever harden by itself. A black smith is hardening metal or a drill sergeant is hardening the soldiers.

Abhärten This is about something or somebody that is hardening himself or herself or itself. A crab is creating a hard skin to protect itself. I'm going for a swim in a winters lake to harden myself against the cold. This is like creating a barrier for any external influence. Once I'm strong/hard enough everything will just peel of me like rain dropping from an umbrella. In German you would say abperlen (water is dropping/dripping off) or abprallen (a weapon bounces back because of my armor). It all uses the prefix ab because it's outside-in.