This invention relates to an envelope and method of making an envelope and, more particularly, to envelopes having a tamper-evident feature to indicate when tampering has occurred after the envelope has been closed and sealed. 
There is a continuous need for containers for the shipment and handling of items, especially valuable items.  In addition to the use of such containers for money, such as the deposit of daily money receipts in a bank's night depository by retail establishments, other items of value must be transferred between parties.  These include checks, bonds, stocks, food stamps, coupons, medical reports and samples, jewelry, confidential documents, etc.  Because of the underlying value of such items, the containers used for such transport and storage should be of high integrity.  The container, in addition to being capable of being handled during transfer without being broken or opened unintentionally to provide access to the contents, must be capable of indicating when its integrity has been compromised. 
Containers of this type are known in the prior art.  For instance, U.S. Pat.  No.  4,483,018 discloses a container alleged to be disposable, of high integrity and tamper resistant.  The container is formed of thermoplastic with an opening to insert contents.  A flap with adhesive is closed over the opening and bonded to the container material.  The flap has a detachable end, and both the flap and the detachable end have identical identification indicia thereon.  The container also has tamper attempt indicating perforations running through the adhesive and indicia parallel to the envelope opening. 
Another such security container is disclosed in European Patent Application No.  85308475.4, published June 4, 1986.  Here a single container used as a security bag has a flap and lip which unite through activation of the adhesive.  By the use of this structure, the bag cannot be opened without severing the material of the bag, thus making the bag tamper evident.  Once an attempt to open the bag is made, it is difficult to realign the row of slits and pilferage becomes apparent. 
A problem has recently been uncovered regarding the use of adhesive type materials to seal plastic envelopes.  It has been found that once the seal has been activated to secure the contents in the envelope, the seal can be reopened, some or all the contents removed and the flap resealed, all without any indication that tampering with the envelope flap has occurred.  This can be carried out by the application of low temperatures to the adhesive region.  For instance, a spray from a can of freon-like material or the application of dry ice to the adhesive region will cause the adhesive to separate from the plastic envelope so that the flap can be lifted off the envelope and access gained to the contents.  After the removal of the low temperature, the adhesive will readily reseal the flap to the plastic envelope without any evidence of tampering. 
The present invention is a new and improved approach to providing a tamper-evident seal for an envelope and method of making such an envelope.