How is the 'z' pronounced in this Dutch name?

[Wednesbury]

Lorentz.

Do you think it is said:

Lorentz (ie with a 'z' sound), or
Lorents (ie with an 's' sound)

The answer determines whether I give it an ' or an 's when used in the possessive form.

Lorentz is a Dutch name (in this context). I've had an inconclusive google. Any Dutch speakers who also happen to be pedants?

Any help welcome!

I'd say an s sound in Dutch, z sound in German.. Lorentz's sounds wrong to me; Lorentz' sounds about right, but maybe looks a little strange

Thanks! I have gone with the 's' sound.

I knew there would be someone helpful around