In a case that has given a rare glimpse of life in South African prisons--normally concealed by a stringent secrecy law--six guards were found guilty today of what a provincial circuit court judge called "an evil day" of brutality against black convicts.

Evidence was presented alleging that the guards, four white and two black, took the convicts to a prison farm where they beat and overworked them in 95 degree heat until 34 prisoners collapsed from injuries and heat exhaustion and three died.

The guards were charged with murder and assault, but prosecutor S.A. Engelbrecht did not press the murder charges, saying he could not prove intent to kill. Judge D.O. Vermooten today acquitted the guards of murder, which carries the death penalty in South Africa, and manslaughter, but he found them guilty of assault.

Two other black guards were acquitted because of the lack of evidence against them.

Vermooten will pass sentence Wednesday after hearing pleas for leniency from defense lawyers.

The case has raised a public storm here following the disclosures of treatment in the prison in the eastern Transvaal town of Barberton. A 24-year-old law prevents the public disclosure of anything that happens in a prison unless it is confirmed by the prison authorities or revealed in an occasional court case like this one.

Adding to the storm is the fact that there have been three violent clashes between guards and convicts, in which eight convicts have been killed, in the same prison during the two months that the trial has been in progress.

Newspapers, which have been prosecuted under the secrecy law in the past for trying to expose brutality in prisons, are blaming the lack of public scrutiny for allowing the prisons to deteriorate.

"Every now and then the system coughs up a corpse, and we catch a glimpse in the court proceedings that follow of the nightmare that reigns behind those high walls," remarked the Johannesburg Sunday Express in an editorial.

The minister of prisons, Jacobus Coetsee, ordered an investigation by his department into conditions at the prison after flying there to inspect it Sept. 21 following the latest outbreak of violence, in which four convicts were killed and three others and two guards were injured.

Part of the problem was that the Barberton prison had "the worst possible element" of the country's convict population, Coetsee said after his inspection.

According to testimony presented in the case, the guards took 44 black convicts, including some who were unfit, sick and crippled, on a "punishment expedition" to a prison farm on a sweltering day in the southern hemisphere summer last December.

There they thrashed them with heavy rubber truncheons, which the judge described as "dangerous weapons," while forcing them to wheel heavy loads of gravel around a work site in the heat.

As they collapsed with injuries and heat exhaustion they were dragged off to what one witness called a "human dump," where one of the guards beat them further as they lay semiconscious in the blazing sun.

"For two hours the work site became a battlefield, leaving three corpses and a large number of injured convicts," Vermooten said in his judgment.

He noted, too, that while the convicts were collapsing one of the senior guards became concerned that the heat might be too much for a guard dog and ordered that it be taken into the shade of a tree.

"I repeat my question why those officials in charge, who were responsible people, were so concerned about a dog when they did not care about the collapsed prisoners who were left lying in the blazing sun," Vermooten said.