EXHIBIT 10 (h)
                                NSC CORPORATION

                               THIRD AMENDMENT TO
                           REVOLVING CREDIT AGREEMENT

THIS THIRD AMENDMENT TO REVOLVING CREDIT  AGREEMENT (this "Third  Amendment") is
made  and  entered  into  as of the  9th  day of May,  1997,  by and  among  NSC
CORPORATION,  a Delaware corporation (the "Parent"),  its Subsidiaries listed on
the signature pages hereto (the "Subsidiaries", the Parent and such Subsidiaries
collectively  referred  to  herein  as the  "Borrowers"  and  individually  as a
"Borrower"),  each of which Borrowers  having its principal place of business at
49  Danton  Drive,  Methuen,  Massachusetts,  01844,  BANKBOSTON,  N.A.  ("BKB",
formerly  known as The  First  National  Bank of  Boston),  a  national  banking
association  having its  principal  place of  business  at 100  Federal  Street,
Boston,  Massachusetts 02110, FLEET NATIONAL BANK ("Fleet"),  a national banking
association  formerly  known  as  Fleet  Bank of  Massachusetts,  N.A.  with its
principal place of business at One Federal Street,  Boston,  Massachusetts 02111
(the "Banks"), and BKB, as Agent for the Banks (the "Agent").

WHEREAS, the Borrowers,  the Banks and the Agent entered into a Revolving Credit
Agreement  dated as of May 4, 1993 and amended as of December 2, 1993 and May 1,
1996 (the "Credit Agreement") pursuant to which the Banks extended credit to the
Borrowers on the terms set forth therein:

WHEREAS, the Banks, the Borrowers, and the Agent have agreed to amend the Credit
Agreement as hereinafter set forth:

NOW, THEREFORE,  for good and valuable consideration the receipt and sufficiency
of which is hereby acknowledged, the parties hereto agree as follows:

I.  AMENDMENT TO THE CREDIT AGREEMENT

A.  Amendment to ss.1.1 of the Credit Agreement.

The following definition  is  hereby  added to ss.1.1 of the  Credit  Agreement:
"Applicable  Commitment  Fee.  The  Applicable  Commitment  Fee set forth in the
following table:

-----------------------------------------------------------
Pricing Ratio                Applicable Commitment Fee
-----------------------------------------------------------
less than 2.00:1                  0.250% per annum
-----------------------------------------------------------
greater than or equal to
2.00:1 but less than 2.50:1       0.375% per annum
-----------------------------------------------------------
greater than or equal to
2.50:1 but less than 3.00:1       0.375% per annum
-----------------------------------------------------------
greater than or equal to 3.00:1   0.500% per annum
-----------------------------------------------------------

The effective  date of a change in the  Applicable  Commitment  Fee shall be the
first day after receipt by the Banks of financial  statements delivered pursuant
to ss.7.4(a) or (b) hereof which  indicate a change in the Pricing  Ratio and in
the   Applicable   Commitment   Fee  in   accordance   with  the  above   table.
Notwithstanding  the  foregoing,   until  receipt  by  the  Banks  of  financial
statements  delivered pursuant to ss.7.4(a) or (b) hereof for the fiscal quarter
ended June 30, 1997, the Applicable  Commitment Fee shall be 0.25% per annum. If
at the time of  calculation  of the  Commitment  Fee the most  recent  financial
statements required to be delivered pursuant to ss.7.4(a) or (b) hereof have not
been delivered, the Applicable Commitment Fee shall be 0.500% per annum, subject
to adjustment upon actual receipt of such financial statements."

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B.  Amendment to ss.5.3(a) of the Credit Agreement.

Section 5.3(a) of the Credit Agreement is hereby deleted in its entirety and the
following substituted in place thereof:

ss.5.3(a)  Commitment  Fee. The  Borrowers  agree to pay to the Agent a fee (the
"Commitment  Fee") in an amount equal to the  Applicable  Commitment  Fee on the
unused  portion of the Total  Commitment  during each fiscal  quarter or portion
thereof from the date of this Third  Amendment  to the Maturity  Date (or to the
date of termination in full of the Total Commitment, if earlier). The commitment
Fee shall be payable in arrears on the first day of each fiscal  quarter for the
immediately  preceding fiscal quarter, with a final payment on the Maturity Date
(or to the date of termination in full of the Total Commitment, if earlier). For
purposes of computing the Commitment Fee, the Maximum Drawing Amount of all L/Cs
shall be considered  usage with respect to the  Commitment.  The  Commitment Fee
shall be shared  among the Banks pro rata in  accordance  with their  Commitment
Percentages.

C.  References to FNBB.

All  references  to the term  "FNBB" or to "The First  National  Bank of Boston"
appearing in the Credit  Agreement and any of the other Loan Documents  shall be
deemed to be references to BKB.

II.  PROVISIONS RELATING TO THIS THIRD AMENDMENT

A.  Definitions.

Capitalized  terms used herein without  definition have the meanings ascribed to
them in the Credit Agreement.

B.  Ratification, etc.

Except as expressly amended or waived hereby,  the Credit  Agreement,  the other
Loan Documents and all documents, instruments and agreements related thereto are
hereby  ratified and confirmed in all respects and shall  continue in full force
and effect.  This Third  Amendment and the Credit  Agreement  shall hereafter be
read and  construed  together as a single  document,  and all  references in the
Credit Agreement or any related  agreement or instrument to the Credit Agreement
shall refer to the Credit Agreement as amended by this Third Amendment.

C.  GOVERNING LAW.

THIS THIRD  AMENDMENT  SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE
LAWS OF THE  COMMONWEALTH  OF  MASSACHUSETTS  AND SHALL TAKE  EFFECT AS A SEALED
INSTRUMENT IN ACCORDANCE WITH SUCH LAWS.

D.  Counterparts.

This  Third  Amendment  may be  executed  in any number of  counterparts  and by
different  parties  hereto  on  separate  counterparts,  each of  which  when so
executed and delivered shall be an original, but all of which counterparts taken
together  shall be deemed to constitute  one and the same  instrument.  Complete
sets of counterparts shall be lodged with the Banks.

E.  Effectiveness.

This Third  Amendment  shall become  effective  upon the  execution and delivery
thereof by the respective parties hereto.

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F.  Entire Agreement.

THE  CREDIT  AGREEMENT  AND THE  SECURITY  DOCUMENTS  AS  AMENDED  BY THIS THIRD
AMENDMENT  REPRESENTS  THE FINAL  AGREEMENT  BETWEEN  THE PARTIES AND MAY NOT BE
CONTRADICTED  BY  EVIDENCE  OF  PRIOR,   CONTEMPORANEOUS,   OR  SUBSEQUENT  ORAL
AGREEMENTS OF THE PARTIES.  THERE ARE NO UNWRITTEN ORAL  AGREEMENTS  BETWEEN THE
PARTIES.

IN WITNESS  WHEREOF,  the  undersigned  have duly executed this Third  amendment
under seal as of the date first set forth above.

THE BORROWERS:

NSC CORPORATION
By:    J. Drennan Lowell
Title:  Vice President

NATIONAL SERVICE CLEANING CORP.
By:  Efstathios A. Kouninis
Title:  Vice President

NATIONAL SURFACE CLEANING CORP.
By:    Greg Weimers
Title:  Vice President

OLSHAN DEMOLISHING MANAGEMENT, INC.
By:  Efstathios A. Kouninis
Title:  Vice President

NSC SPECIALTY COATINGS, INC.
By:  Efstathios A. Kouninis
Title:  Vice President

NSC ENERGY SERVICES, INC.
By:  Efstathios A. Kouninis
Title:  Vice President



THE BANKS:

BANKBOSTON, N.A.
(formerly known as The First National
Bank of Boston)
By:
Title:

FLEET NATIONAL BANK
By:
Title:

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