Existing Plaster -- Gypsum or Cement?

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Author Message David Stutzman New member Username: David_stutzman Post Number: 4 Registered: 07-2002 Posted on Tuesday, March 25, 2003 - 01:51 pm:    We are remodeling an historic building approximately 80 years old located in Missouri. We would like to match the existing plaster but are not sure of the type. The existing plaster is lightweight (low density) and finished with a slight pebble surface. The plaster holds up to complete saturation and drying, leading me to believe it is portland cement instead of gypsum. I asked a gypsum plaster manufacturer for a simple test to determine which type the plaster is. Apparently there is no simple test. Any suggestions, ideas, or other resources? Dave Metzger New member Username: Davemetzger Post Number: 33 Registered: 07-2001 Posted on Tuesday, March 25, 2003 - 03:10 pm:    It's not only important to determine the plaster type, but also that the new plaster mix is compatible with existing. You could have a sample of the existing plaster tested at a material testing lab to determine mix proportions, aggregate type, etc. The materials and mixes used in repair Work need to be compatible with existing materials and mixes, in order to provide comparable physical characteristics to preclude differential shrinkage, movement or color variation which would impair repair Work, including cracking or spalling between repaired and existing Work. David Stutzman New member Username: David_stutzman Post Number: 5 Registered: 07-2002 Posted on Tuesday, March 25, 2003 - 03:38 pm:    The repair is more a wholesale type rather than localized. So the compatibility is not quite so critical since whole walls or ceilings will be replaced within a room. The idea is to replce the plaster with a like kind. I learned that a local plasterer has determined that it is gypsum plaster because he was able to break a corner off of the sample. Now there is a definitive, scientific test. No mention if the sample was fractured during removal. William C. Pegues, FCSI, CCS New member Username: Wpegues Post Number: 124 Registered: 10-2002 Posted on Tuesday, March 25, 2003 - 11:25 pm:    Dave S, Hi there, long time no see! There is of course the taste test -grin! William David Stutzman New member Username: David_stutzman Post Number: 6 Registered: 07-2002 Posted on Wednesday, March 26, 2003 - 11:02 am:    William, So what are the taste differences? I still have the sample, so I will get right to it. I'll let my staff know whose idea it was, just in case. Hope to see you in Chicago. Dave John Regener, AIA, CCS, CCCA, CSI New member Username: John_regener Post Number: 77 Registered: 04-2002 Posted on Wednesday, March 26, 2003 - 11:13 am:    William: Taste test? Is that like, "It's not just a floor wax but a great, low-calorie dessert topping." Dave Metzger New member Username: Davemetzger Post Number: 34 Registered: 07-2001 Posted on Wednesday, March 26, 2003 - 11:31 am:    You guys are just drunk (dare I say plastered) with your cleverness, aren't ya? David Stutzman New member Username: David_stutzman Post Number: 7 Registered: 07-2002 Posted on Wednesday, March 26, 2003 - 06:54 pm:    I'm glad I oculd bring some levity to this forum. You have all been a great help!! Tracy Van Niel New member Username: Tracy_van_niel Post Number: 28 Registered: 04-2002 Posted on Thursday, March 27, 2003 - 09:22 am:    Speaking as someone who didn't eat plaster when I was young, I'm curious too as to what the taste difference is between gypsum and portland cement plaster. For those of you attending the taste test in Chicago, could you share your findings when you get back? Another thought on plaster that's 80 years old ... don't you also have to be concerned about asbestos in the plaster? John Bunzick, CCS, CCCA New member Username: Bunzick Post Number: 83 Registered: 03-2002 Posted on Thursday, March 27, 2003 - 12:03 pm:    Yes, old plaster does sometimes contain asbestos, unfortunately. As to gypsum versus portland cement, it would seem to me that interior plaster is pretty unlikely to by portland cement. And if it was, would you really want to replace it with portland cement rather than gypsum? (or is it being patched.) I'm not a chemist, but it would seem to me that it should be quite easy to test for the type of binder without doing a complete analysis like would be done with masonry mortar. I would expect that portland cement plaster would be denser and harder than gypsum, but that's too subjective to be of much use. I'm not sure that the wetting and drying really indicates too much. Gypsum can be wetted and dried without disolving, unlike today's ready-mix vinyl joint compound. Richard Hird Unregistered guest Posted on Thursday, March 27, 2003 - 02:09 pm:    Very old plaster contains horse hair. Does anyone know the difference in taste between horse hair and asbestos? Ron Beard CCS New member Username: Rm_beard_ccs Post Number: 16 Registered: 10-2002 Posted on Thursday, March 27, 2003 - 04:45 pm:    Which end of the horse? David Stutzman New member Username: David_stutzman Post Number: 8 Registered: 07-2002 Posted on Thursday, March 27, 2003 - 08:13 pm:    Just to remove fear of tast testing, there is no fiber evident: asbestos or hair. I had my share of patching horse hair plaster on wood lath in circa 1840 buildings. So I can rule that out. I think I'll bring the sample to SCIP for all to taste test. This plaster was installed on metal lath - the imprint and rust are still there. Those of you daring enough may be able to get your MDA for iron. I'm surprised plaster was used at all since the building is a college dorm; although I don't know if that was the original use. By today's standards I would expect concrete or CMU. John Bunzick, CCS, CCCA New member Username: Bunzick Post Number: 85 Registered: 03-2002 Posted on Friday, March 28, 2003 - 08:35 am:    Just as a note: you cannot detect asbestos fibers with the naked eye. Their size, combined with low concentrations, would make them invisible. Ron Beard CCS New member Username: Rm_beard_ccs Post Number: 17 Registered: 10-2002 Posted on Friday, March 28, 2003 - 09:23 am:    John: Asbestos fibers processed through a manufacturing process can get very small but they, as a general rule, are visible. They can be seen in products like the old fiberboard ceiling tiles and in some of the old loose insulations. Years ago in the Washington DC area construction was halted on a major shopping mall when they dug into a natural deposit of asbestos. I've seen mason jars filled with twisted asbestos strands about 6" to 8" long and about 1/2" in diameter in size from that project. John Bunzick, CCS, CCCA New member Username: Bunzick Post Number: 86 Registered: 03-2002 Posted on Friday, March 28, 2003 - 10:00 am:    Ron, I agree that in its mineral form, such as you describe coming out of the ground, it would be visible. When I was a kid I had a "mineral collection" which included a small chunk of asbestos! It was really neat to see that bundle of long, parallel fibers - very fascinating. Hopefully the amount I inhaled was not meaningful. However, I still am not sure that the fibers you can see in fiberboard ceilings are asbestos, as opposed to other fiber types that are used in these products. The concentrations of asbestos fiber by weight were not that high, and the fibers are extremely short in length once they are processed into a building product. I think my point is that seeing or not seeing fibers is not a reliable indicator of the presence of asbestos. Knowing the types of products that historically contained asbestos is probably the best indicator of whether one should test. At least I wouldn't rely on my ability to determine if the fibers I saw were asbestos. William C. Pegues, FCSI, CCS New member Username: Wpegues Post Number: 125 Registered: 10-2002 Posted on Tuesday, April 01, 2003 - 01:19 pm:    Wow, so many people like eating plaster! I would never have guessed. I will have to remember this and pass it on to my chapter for planning the menu at future meetings! Ron, that was Fair Oaks, but it was not the mall that was temporarlily halted, it was a large residential construcction just across the highway that got really delayed. There were some retail areas nearby but Fair Oaks mall itself was not bothered. Northern Virginia have lots of natural deposites of asbestos...as well as radon gas. William Ron Beard CCS New member Username: Rm_beard_ccs Post Number: 18 Registered: 10-2002 Posted on Tuesday, April 01, 2003 - 04:36 pm:    William: With all that fiber and gas, I'm not sure that I want to eat at one of your meetings! <G> Please don't start with the May meeting.