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PoP dryer re-visited

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#1 Vodkaman


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Posted 18 March 2009 - 02:56 AM

So why air dry?

There are arguments for and against oven drying Plaster of Paris molds. A lot of the TU Plaster of Paris artists use oven drying, so it cant be all bad. Providing that the temperature does not exceed 150C. At this temperature the gypsum (cured mold) is converted back to plaster of Paris, resulting in a powdery mold and is spoilt. Professionals use ventilated ovens, set at 125F (52C).

For arguments for and against and any other information, Google will provide you with all the answers. But I have no choice, as I have no oven in my kitchen. Maybe your wife is not too impressed about you cooking building materials in her prized possession. Regardless of the pros and cons, this thread is about the alternatives.

Further to my thread on drying Plaster of Paris, a while back http://www.tackleunderground.com/forum/homebrew-tools/11262-pop-dryer.html I did some tests on various methods of air drying Plaster of Paris, for comparison. I had to keep the Plaster of Paris samples small, as I intended to monitor the weight and my digital scale only goes to 300gm. Although the test pieces are small compared to your average mold, the results are still relevant and can even be scaled up to predict results for larger molds.

The four methods of drying that I tested, were:

1. air drying in a warm room. As I am living in Indonesia, the room temperature is relatively high, between 26C and 29C, also, the humidity is fairly high.

2. behind the refrigerator, towards the top of heat exchanger, mounted on a temporary shelf, with the refrigerator close to a wall. Utilizing heat from the heat exchanger combined with the updraft of warm air rising.

3. drying box fitted with fan, no heat source.

4. drying box fitted with fan and regular 75W bulb as a heat source.


I monitored the elapsed time and weight of the samples. From these figures I also calculated the weight loss per hour and the percentage loss. I de-molded the samples about 30 minutes after mixing the Plaster of Paris and started the test. All the samples were identical, I used a gram scale to weigh out the water and Plaster of Paris and used the same mixing process for all.

Results and findings.

All the samples maxed out with a loss of weight of 35% of the start weight.

All the samples lost weight at constant rates after the first hour, down to 30% loss, after which, the weight loss per hour reduced.

Room temperature and humidity changes had significant impact on the results. For example, a storm with its associated rise in humidity, slowed down the weight loss rate, as did the cooler temperatures of the night.

I measured the dryer box and room temperatures. At a room temperature of 28.1C with the lamp off, the mold temperature was 27.6C. with the lamp on, the mold temperature was 30.3C. This leaves lots of room for increasing the temperature, to achieve faster times.

drying method_____30% loss time_____loss per hour
air drying_________96_______________0.65
box and fan_______13________________4.5
box,fan and lamp___7.5_______________9.0

Extrapolating the data to a normal size mold, say 9 x 5 x 1.5

drying method_____30% loss time_____loss per hour

air drying____________252____________2.5
box and fan___________36____________17.2
box,fan and lamp_______18____________34.4


Anything less than 24 hours for drying a mold seems reasonable to me. But a mold drying box, large enough to take your largest mold, with an inch of air space all round, is also large enough to carry two fans. Also, by increasing the wattage and/or increasing the number of bulbs, this time could be halved or even better.

Test drying box

The fan is 6 square, mains voltage, 0.14 amps, so very low cost to run. At the end opposite to the fan (see picture), can be seen two packing plates. These constrict the air gap around the Plaster of Paris, to about 1. Above the packers, is the mold shelf fitted with dowels. The Plaster of Paris mold sits on the dowels, thus allowing the air to circulate around the entire mold. The packers, mold shelf are pushed inside the dryer box, followed by the lamp(s). The fan pulls air through the box.

Improvements to the prototype would be two lids, to access the molds and the lamps. A vast improvement could be made by a closed loop, recycling the same air. A humidity sensor hooked up to an actuated door would open every few minutes to vent the wet air. This would greatly increase the working temperature for less power and reduce drying times to comparable with an oven or better, due to the volume of circulating air. For the use of the average TUist, this would be a shade over the top, but if you make molds on a daily basis, it could be worth while.


Edited by Vodkaman, 18 March 2009 - 03:28 AM.
word tabulations did not work, now its scruffy!!!

#2 redg8r



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Posted 18 March 2009 - 06:16 AM

That reminds me of a woodworkers lamp kiln. They work well when made and monitored safely.

Great article Dave, should work well for you.

#3 HJS



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Posted 30 March 2009 - 08:09 AM

Excellent study concerning speed of drying of Plaster of Paris using different drying methods.

There's another issue I've been wondering about... and that is does drying time affect the final hardness of molds made from the Plaster of Paris or DWP? Is slower drying (days) result in a harder mold than those force-dried in an oven (couple hours)? I think tyhis is pretty much what you were getting at with your study.

I know for a fact that when highway guys are pouring a new concrete deck on a bridge, they like the concrete to dry as slowly as possible to make it harder. During the warmer months they will go so far as to cover the setting concrete with wet burlap to slow the evaporation/hardening process... slower = harder. Boat hauls are sometimes made from concrete too and the setting concrete will be sprayed with a mist of water for days to slow the curing/drying process. All done to make the concrete haul harder.

Back to lure molds. Minor flaking of the lure cavity edges can sometimes be a big issue with my DWP 2-piece molds. Might slower=harder concept as it applies to concrete also apply to the drying time of DWP or Plaster of Paris??? Hmmmm.....

#4 nova


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Posted 30 March 2009 - 10:58 AM

I knew there was another reason that I didn't feel comfortable setting my oven higher than 150. My concern was that I didn't want to "steam" the water and crack the molds.

I also take my molds out of the oven a couple of times and set them on a folded newspaper to draw out some of the water.

Great article Dave.


#5 Vodkaman


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Posted 30 March 2009 - 01:57 PM

"Gamers" web sites contain a lot of information and research into Plaster of Paris, its variations and additives. This is a link to one such site: Construction Corner it further links other related information.

There is plenty of evidence that the mixing proportions are important. Too thin and the plaster mold will be weak. This link has a good section on mixing plaster: Hirst Arts Casting Page

This article has an interesting section on release agents: JAIC 1998, Volume 37, Number 1, Article 4 (pp. 35 to 47) see last para of section 3.2 and section 3.3.

This link contains a couple of comments relative to the oven drying weakness problem, but it is only one mans opinion: Re: ATM Does a *common plaster of Paris* tool deform when wet?

This link has a reference to mixing proportions by weight, which is the way I do mine. It suggests a ratio of 60 (Plaster of Paris) / 40 (water), I actually use a slightly thinner ratio mix of 5/4 by weight. http://dspace.unimap...Methodology.pdf

The third paragraph of this link: Re: ATM Plaster of Paris Tools suggests letting the mold cool down slowly in the oven, as removing the Plaster of Paris cast to a cold climate could crack the mold. Seems a reasonable suggestion to me.

In summary, I could not find anything that says that oven drying Plaster of Paris is a bad thing, other than keeping the temperature well below 150C.

I found a few references to reinforcing the mix with various materials such as wood pulp, hessian, etc. but more importantly, the mix has to be right, too weak a mixture is a bad thing.


Edited by Vodkaman, 30 March 2009 - 01:59 PM.