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A powdered form of tannic acid extract from the bark of the quebracho tree, used as a high-pH and lime mud deflocculant. It was in widespread use until the 1950s, at which time lignosulfonate became widely available and performed the same function better and cheaper than quebracho. High pH was needed to neutralize tannic acids to form the tannates, which are red. The oilfield name red mud was applied to tannate-dispersed muds.
A legend of the oil patch holds that, back in the days when quebracho mud was all the rage (in the days when Hobby Airport was Houston's main airport), a mud man parked his old field car in the airport parking lot for several days. It was raining when he left, but sunny when he got back. The police were waiting to talk to him when he returned because someone noticed a dried blood-red stain under his car. The liquid had obviously leaked out of the trunk. When the mud man opened the trunk for the police, they found two wet sacks of quebracho. As you may have guessed, his trunk lid leaked.