It is one of the kalpana in bhaishajyashatra.

द्रव्यमार्द्र शिलापिष्टं शुष्कं वा सजलं भावेत् /
प्रक्षेप आवाप कल्कास्ते तन्मानं कर्षसाम्मिताम् // शा म ५/१

Kalka means soft mass or bolus of herbs prepared by pasting them with the help of mortar and pestle.At the time of preparation some amount of water should be added according to requirement.Kalka of Fresh and soft herbs can easily obtained.for preparing kalka from fresh herbs,it should be cuts in to small pieces first.

But In case of dry hebs ,the part of dry herbs has to be soaked in water for some period of time,and allow to absorbs water till it become soft then this part is grounded in mortar and pestle to obtain fine paste.

यः पिण्डो रसपिष्टनां स कल्कः परिकीर्तितः // च सु ४/७

According to Acharya Charaka,kalka should be obtained by pasting of fresh herbs on mortar and pestle.The medicine obtained by this method is in bolus form.This bolus is neither too dry nor too liquid in consistency.
Exact water quantity should be maintained while preparing the kalka .
Fibrous part of herbs should be removed while preparing the kalka .
When fresh dravya or herbs not available ,then fine powder of dried herbs should use.This is mixed with appropriate quantity of water and then rubbed in pestle and mortar to made in to paste.

Kalka kalpana can be used for external as well as internal application.
It is laghu and easy to digest than swarasa kalpana
It is also known as Prakshepa or Aavapa

Prakshepa Dravya-

कल्के मधु घृतं तैलं देयं द्विगुणमात्रया /
सितगुड समं दद्यात् द्रवा देयाः चातुर्गुणः // शा म ५/२

The prakesha dravya like Madhu (honey),ghreet (ghee) and taila (oil) should be added in double quantity of kalka.
Sita (sugar candy) and guda(jaggery) should be added in equal quantity of kalka dravya,while drava (liquid) should be added four times to that of kalka.

Dosage-
Dosage of Kalka is 1 karsha (about 12 gm) and it has to be consumed immediately after preparation.
For exernal application, according to area of application,quantity of kalka dravya is used.

Saviryata Avadhi-24 hours

Some Examples-
Nimba patra kalka used in intestinal worm infestation
Rason kalka –used in vatavyadhi
Importance Of Kalka Kalpana-
Use to cures the disease by internal and external application
Used in the preparations of secondary kalpanas like vati kalpana,sneha kalpana