Methylene blue clay determination: which procedure's correct.

Methylene Blue Clay Determination: Which Procedure's Correct?

As industries adopt new testing methods, there is always a tendency to modify or change the test procedure to meet the individual's goal. These changes often result from the plant or laboratory not having the proper testing equipment, requirements to shorten or speed up the test, or the belief that their product is unique and needs to be tested differently.

This same tendency to "change the process" has entered into the testing procedures for active clay content determination via methylene blue. Unfortunately, as these changes are incorporated into quality assurance measures, the results obtained cannot be correlated with other facilities or suppliers.

The Casting Industry Supplier's Association joined with the American Foundrymen's Society to determine the magnitude of these variations in methylene blue test procedures and to restandardize the test procedure for the industry. CISA's method of approach was to request and obtain the cooperation of eight highly recognized laboratories. Each laboratory agreed to perform ten repetitions of their current methylene blue procedures on four samples of unknown products. These products included "A" 100% western bentonite, "B" 100% southern bentonite, "C" 50% western/50% southern bentonite, and "D" a pre-blended foundry mixture. The laboratories were also asked to send a detailed copy of their procedure so that a determination of the degree of procedure variance could be completed.

The major variations in procedures included:

1. Sample sizes ranging from 0.3 grams to 1.0 grams.

2. Ultrasonic scrubbing for five minutes or seven minutes, boiling for ten minutes or using a magnetic stirrer for the clay dispersion with no ultrasonic or mixer used.

3. Reporting formats which indicated active clay percent or methylene blue uptake in milliliters.

4. Varied procedures in methylene blue and tetrasodium pyrophosphate preparation procedures.

Due to the variances in testing procedures and solution formulations, the data obtained varied significantly (Table 1).

After correlating the data and noting the variances in procedures, the participants were asked to send their technical representatives to AFS headquarters to develop a standard methylene blue procedure for the foundry industry. The procedures agreed upon allows the use of either ultrasonic scrubbing or boiling of the sample with the notation that the boiling method will generally provide one to four milliliters higher uptake than the ultrasonic method. A significant amount of time during the meeting focused on detailing the proper method of preparing the methylene blue and tetrasodium pyrophosphate solutions.

The new procedure was presented to AFS and has been included in the AFS Core and Mold Test Handbook (1989). The new procedures are shown on the facing page.

After establishing the new procedure the eight laboratories were asked to participate in a second round of testing to determine if the new procedures reduced the variances previously observed. The second round was limited to the 100% western bentonite and the 50% western/50% southern blend. Each laboratory was requested to repeat each of the tests five times. One of the laboratories continued to use the 1.0 gram sample size for their analyses. This laboratory used the boiling method of analysis and felt the method obtained more consistent results with the larger sample.

The results of the second round of testing are shown in Table 2.

NOTE: During the organization and publishing of the new AFS "Mold and Core Test Handbook," a procedure labeled "The Methylene Blue Clay Test-Boiling Method, Procedure 211-87-S" was inadvertently included. This is not the boiling method referred to in this article and should not be used for determination of the methylene blue.

CISA will continue to assist AFS in establishing testing procedures and insuring that the suppliers to the casting industry help to stabilize the foundry testing procedures. Future studies are already progressing on moisture and loss on ignition testing for clays and preblends. These procedures will be reported later.

The Casting Industry Supplier's Association would like to thank the following companies for their assistance in completing this study;

Aimcor, American Colloid Company, Bentonite Corporation, George Fischer Foundry Systems, The Hill & Griffith Company, J. S. McCormick Company, M. A. Bell, Wedron Silica Company and Whibco.