Hi Jean,

This is a very interesting question. 

Clore and 'Éclore' don't have an imperfect, a past historic and are part of verbs which are called 'defective' ( défectif in French) by the very reason that they cannot be conjugated like other verbs as they have missing tenses or forms.

' Falloir' and 'pleuvoir' are other examples as they are only conjugated in the third person singular like all impersonal verbs.

Others include, absoudre , accroître, faillir ....

This is what the Académie Française has to say about them - 

http://www.academie-francaise.fr/verbes-defectifs-je-closis-et-je-traisis-il-appert

Hope this helps!