Agreement was reached yesterday to temporarily halt the acquisition of Chemetron Corp., the organic pigments business of Allegheny Ludlum Industries, Inc., by Rhinechem Corp.

In a stipulated order filed in U.S. District Court in Chicago and approved by Judge John F. Grady, lawyers for the Federal Trade Commission, and the two companies agreed that Rhinechem would not acquire any of Chemetron's assets until 48 hours after the district court has ruled on the FTC's motion for a preliminary injunction. The FTC is challenging the legality of the merger.

Under the terms of the order approved yesterday a hearing has been scheduled for Sept. 5 before U.S. District Judge Joel M. Flaum in Chicago to set a date for a hearing on the preliminary injunction and to hear other motions.

The FTC last week challenged the proposed merger on grounds that it would decrease competition in the organic pigments market. At the same time, the FTC instructed its staff to seek an injunction that would prevent the merger from being consummated while its legality was being tested in as administrative hearing at the FTC.

Organic pigments are used to impart color to a variety of products and are said to be more brilliant in color and possess a much broader range of colors and shades than inorganic pigments.

According to the FTC, Rhinechem, through its wholly-owned subsidiary. Harmon Colors Corp., was the eighth largest manufacturer of organic pigments in the U.S. with a market share of about 6 percent. Rhinechem is a wholly-owned subsidiary of Bayer International Finance N.A., which is in turn a wholly-owned subsidiary of Bayer AG of West Germany.

Chemetron was the third largest concern in the industry with a market share of 11 percent, the FTC said.

According to the complaint, the proposed combination of the two companies may substantially lessen competition or tend to create a monopoly in the manufacture and sale of organic pigments. Actual competition between the two compaines would be eliminated and the level of concentration in the industry would be increased, the FTC alleged.

The companies entered into an agreement in principle in June under which Rhinechem agreed to pay cash for Allegheny Ludlumn's organic pigments division and would assume "certain liabilities." Further terms were not disclosed.