Exhibit 10.35

                               SECURITY AGREEMENT

      THIS SECURITY AGREEMENT (the "Agreement"), dated this 9th day of February,
2001, by and between Tremont Corporation,  a Delaware  Corporation  (hereinafter
called  "Pledgor"),  whose  principal  office is at 1999  Broadway,  Suite 4300,
Denver,  Colorado 80202 and NL Environmental  Management  Services,  Inc., a New
Jersey  Corporation (the "Secured Party"),  in its capacity as the holder of the
Note (as defined below).

      Section 1. Security Interest. For value received, Pledgor hereby grants to
Secured  Party,  upon the terms and  conditions  of this  Agreement,  a security
interest in and to any and all present or future rights of Pledgor in and to all
of the following  rights,  interests  and property  (all of the following  being
herein sometimes called the "Pledged Shares"):

            10,215,541 shares of NYSE-traded NL Industries, Inc. ("NL") common
        stock, par value of $1.25 per share.

As used in this  Agreement,  the  "Collateral"  shall include the Pledged Shares
together  with  any  and  all  products  and  proceeds  of the  Pledged  Shares,
including, without limitation, all dividends, cash, instruments,  subscriptions,
warrants and any other rights and options and other  property  from time to time
received,  receivable or otherwise  distributed in respect of or in exchange for
any or all of the Pledged Shares.

      Section 2. Loan Agreement.  This Agreement is being executed and delivered
pursuant to the terms,  conditions and  requirements  of that certain  revolving
note, dated as of February 9, 2001 ("Revolving Note"), pursuant to which Secured
Party has  loaned  monies  to  Tremont  Corporation  ("Tremont").  The  security
interests herein granted  ("Security  Interests")  shall secure full payment and
performance  of: (a) that certain  Revolving  Note of even date  herewith in the
principal  amount of  $13,400,000,  made by Tremont  and payable to the order of
Secured Party (such note and any notes given in modification, renewal, extension
or substitution thereof being herein sometimes  collectively  referred to as the
"Notes" and individually as the "Note"); and (b) the due and punctual observance
and performance of each and every agreement, covenant and condition on Pledgor's
part to be observed or performed  under this Agreement or the Note (all of which
debts, duties, liabilities and obligations hereinbefore described and covered by
this Agreement and the Note are hereinafter referred to as the "Obligation").

      Section 3.  Priority.  Pledgor  represents  and warrants that the Security
Interests  are  first  and  prior  security  interests  in  and  to  all  of the
Collateral, subject to the following liens thereon or security interests therein
in existence prior to the Pledgor's acquisition of the Collateral:

                                     [NONE]

      Section 4. Title to Collateral. Pledgor represents and warrants to Secured
Party that: (a) Pledgor is the owner of the Collateral; (b) no dispute, right of
offset,  counterclaim,  or defense to the Security Interests exists with respect
to all or any part of the Collateral; and (c) Pledgor will defend the Collateral
against the claims and demands of all persons other than any subordinate  claims
or liens acknowledged by Secured Party.


<PAGE>




      Section  5.  Pledgor's  Obligations.  So long as the Note is  outstanding,
Pledgor  covenants and agrees with Secured Party: (a) not to permit any material
part of the  Collateral  to be levied upon under any legal  process;  (b) not to
dispose of any of the  Collateral  without the prior written  consent of Secured
Party;  (c) to comply with all  applicable  federal,  state and local  statutes,
laws, rules and regulations,  the noncompliance with which could have a material
and  adverse  effect  on the value of the  Collateral;  and (d) to pay all taxes
accruing  after the Closing Date which  constitute,  or may  constitute,  a lien
against the  Collateral,  prior to the date when  penalties  or  interest  would
attach to such taxes;  provided,  that Pledgor may contest any such tax claim if
done diligently and in good faith.

      Section  6. Event of  Default.  As used  herein,  the term  default  shall
include any or all of the following:

            (a) The assignment,  voluntary or involuntary conveyance of legal or
      beneficial interest,  mortgage,  pledge or grant of a security interest in
      any of the Collateral; or

            (b) The  filing or  issuance  of a notice of any lien,  warrant  for
      distraint or notice of levy for taxes or assessment against the Collateral
      (except  for those which are being  contested  in good faith and for which
      adequate reserves have been created); or

            (c) Nonpayment of any installment of principal or interest under the
      Notes; or

            (d) The  adjudication  of Tremont as bankrupt,  or the taking of any
      voluntary  action by Tremont or any  involuntary  action  against  Tremont
      seeking an  adjudication  of Tremont as bankrupt,  or seeking relief by or
      against Tremont under any provision of the Bankruptcy Code; or

            (e) Pledgor  failing to comply with any other covenant  contained in
      the Notes or this Agreement; or

            (f)  Pledgor's  default  in any  payment  (regardless  of amount) of
      principal of or interest on any other indebtedness for borrowed money; or

            (g) Pledgor's  default in the observance or performance of any other
      agreement  or  condition  relating  to any  such  other  indebtedness  for
      borrowed  money or contained  in any  instrument  evidencing,  securing or
      relating  thereto or any other event shall occur or condition  exist,  the
      effect of which  default or other event or  condition  is to cause,  or to
      permit the holder of the  indebtedness to cause,  such other  indebtedness
      for borrowed money to become due prior to its stated maturity.

An "Event of  Default"  shall be deemed to have  occurred  immediately  upon any
default  described  in clause  (d) or (g) above,  if any  default  described  in
clauses  (c) or (f)  above  is not  cured  within  5  days,  and if any  default
described in clauses (a),  (b), or (e) is not cured within 30 days after written
notice from Secured Party to Pledgor.


                                       -2-

<PAGE>



      Section 7. Remedies. Upon the occurrence and during the continuation of an
Event of Default as defined herein,  in addition to any and all other rights and
remedies  which Secured Party may then have  hereunder or under the Note,  under
the Uniform Commercial Code of the State of New Jersey or of any other pertinent
jurisdiction (the "Code"),  or otherwise,  Secured Party may, at its option: (a)
reduce its claim to judgment or  foreclosure  or otherwise  enforce the Security
Interests,  in whole or in part, by any available judicial procedure;  (b) sell,
or otherwise  dispose of, at the office of Secured Party,  or elsewhere,  all or
any part of the Collateral,  and any such sale or other  disposition may be as a
unit or in parcels, by public or private proceedings,  and by way of one or more
contracts (it being agreed that the sale of any part of the Collateral shall not
exhaust the Secured  Party's  power of sale,  but sales may be made from time to
time,  and at any time,  until all of the  Collateral has been sold or until the
Obligation has been paid and performed in full);  (c) at its discretion,  retain
the Collateral in satisfaction of the Obligation  whenever the circumstances are
such that Secured  Party is entitled to do so under the Code or  otherwise;  and
(d) exercise any and all other rights, remedies and privileges he may have under
the Note and the other documents defining the Obligation.

      Section 8.  Application of Proceeds by Secured Party. Any and all proceeds
ever  received  by  Secured  Party  from any sale or  other  disposition  of the
Collateral,  or any part thereof,  or the exercise of any other remedy  pursuant
hereto  shall be applied by Secured  Party to the  Obligation  in such order and
manner  as  Secured  Party,  in  its  sole  discretion,  may  deem  appropriate,
notwithstanding  any  directions  or  instructions  to the  contrary by Pledgor;
provided that the proceeds and/or accounts shall be applied toward  satisfaction
of the Obligation.  Any proceeds  received by Secured Party under this Agreement
in excess of those  necessary  to fully and  completely  satisfy the  Obligation
shall be distributed to Pledgor.

      Section 9. Notice of Sale.  Reasonable  notification of the time and place
of any public sale of the  Collateral,  or reasonable  notification  of the time
after which any private sale or other intended  disposition of the Collateral is
to be made, shall be sent to Pledgor and to any other persons entitled under the
Code to notice;  provided,  that if any of the  Collateral  threatens to decline
speedily  in  value or is of a type  customarily  sold on a  recognized  market,
Secured Party may sell,  pledge,  assign or otherwise  dispose of the Collateral
without  notification,  advertisement  or other notice of any kind. It is agreed
that  notice  sent or given not less than ten (10)  calendar  days  prior to the
taking of the action to which the notice relates is reasonable  notification and
notice for the purposes of this paragraph.

      Section 10.  Right to Vote Collateral; Receipt of Dividends, Etc.

      (a) Unless an Event of Default shall have occurred and be continuing,  the
Pledgor shall have the right,  from time to time, to vote and to give  consents,
ratifications and waivers with respect to the Collateral,  and the Secured Party
shall, upon receiving a written request from the Pledgor, which request shall be
deemed to be a  representation  and  warranty  by the  Pledgor  that no Event of
Default has occurred and is continuing,  deliver to the Pledgor or, as specified
in such request, such proxies, powers of attorney,  consents,  ratifications and
waivers in respect of any  Collateral  which are  registered  in the name of the
Secured  Party or a nominee as shall be specified in such request and be in form
and substance satisfactory to the Secured Party.


                                       -3-

<PAGE>



      (b) If an Event of Default  shall have  occurred  and be  continuing,  all
rights of the Pledgor to exercise the voting and other  consensual  rights which
it would  otherwise  be entitled  to  exercise  pursuant to Section 10 (a) above
shall end upon five days'  notice  from the  Secured  Party to the  Pledgor  and
thereafter  the Secured  Party shall have the right to the extent  permitted  by
law,  and the  Pledgor  shall  take  all  such  action  as may be  necessary  or
appropriate  to  give  effect  to such  right,  to  vote  and to give  consents,
ratifications  and  waivers,  and take any  other  action  with  respect  to all
Collateral  with the same  force and  effect as if the  Secured  Party  were the
absolute and sole owner thereof.

      (c) Unless an Event of Default shall have occurred and be continuing,  the
Pledgor shall be entitled to receive all regular  quarterly cash dividends.  Any
other  dividends  or  distributions  or  proceeds  therefrom  on  account of the
Collateral  shall,  if  received  by the  Pledgor,  be received in trust for the
benefit of the Secured Party,  be segregated from the other property or funds of
the Pledgor,  and be forthwith  delivered to the Secured  Party as collateral in
the same form as so received (with any necessary endorsement).

      Section 11. Delivery of Notices. Any notice or demand required to be given
hereunder  shall be in  writing  and shall be deemed to have been duly given and
received, if given by hand, when a writing containing such notice is received by
the person to whom addressed or, if given by mail, two (2) business days after a
certified or registered letter containing such notice,  with postage prepaid, is
deposited in the United States mails, addressed to:

            If to Secured Party:

            NL Environmental Management Services, Inc.
            16825 Northchase Drive
            Suite 1200
            Houston TX 77060
            Attn:  Vice President and Secretary

            If to Pledgor:

            Tremont Corporation
            1999 Broadway
            Suite 4300
            Denver, Colorado 80202
            Attn:  General Counsel

Any such address may be changed from time to time by serving notice to the other
party as above  provided.  A business  day shall mean a day of the week which is
not  a  Saturday  or  Sunday  or  a  holiday   recognized  by  national  banking
associations.

      Section 12. Binding Effect.  This Agreement shall be binding upon Pledgor,
its successors and assigns, and shall inure to the benefit of Secured Party, its
heirs, successors,  assigns,  executors,  administrators,  and personal or legal
representatives.


                                       -4-

<PAGE>



      Section 13. Governing Law. This Agreement shall be construed in accordance
with and governed by the laws of the state of New Jersey.

      Section  14.  Severability.  In the  event  that  any  one or  more of the
provisions  contained  in this  Agreement  are held to be  invalid,  illegal  or
unenforceable in any respect,  such invalidity,  illegality or  unenforceability
shall not affect any other provision of this Agreement.


                                       -5-

<PAGE>


      EXECUTED as of the day and year first herein set forth.



SECURED PARTY:


NL Environmental Management Services, Inc.

By:      /s/ Robert D. Hardy
         ---------------------------------
         Robert D. Hardy

Title:   Assistant Treasurer
         ---------------------------------


PLEDGOR:


Tremont Corporation

By:      /s/ Robert E. Musgraves
         ---------------------------------
         Robert E. Musgraves

Title:   Vice President
         ---------------------------------



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