Clay, Application to Refinement

 

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Clay Oven (Hot Box)

 

When it's time to start your clay project or any model that involves the use of Automotive clays or Industrial clay as they're called you have to remember that they have to be heated in an oven. This type of oven is not what you would cook your Sunday roast in but more like a warming oven that will keep a constant lower temperature. The more readily used commercial ovens can cost upwards of $2000.00 for a full stainless steel cabinet but this type of expense is not what you would want to pay for a garage type project.

For this purpose and for later use on my Corvette project I set about making a relatively cheap clay box that would enable anyone to build and use on a small scale basis. This quickly made Styrofoam™ box will allow up to three billets of clay to be warmed at a time with a reasonable turn around rate.

Styrofoam box for heating small amounts of clay.

The first image on the left gives the general size that I was able to make from the off cuts of Styrofoam™ that I had. The foam thickness is 1.5 inches (38.0mm) with the inside dimensions of 22" x 13" x 10" I was able to fit a baking tray 13' x 9" x 2"

Above the box is a very inexpensive clamp on lamp that is available from most hardware stores and D.I.Y centers. For this purpose I removed the clamping mechanism as I will be permanently fitting the lamp into the top.

The first course of action for the hot box was to decide how I would hold the pieces together. I had used resin before and that would be OK providing it was a slow cure and did not generate too much heat. High heat from quick curing resins would melt the foam so that was out of the question but I didn't want to wait a long time for a slow cure. 

Contact adhesive such as 3M's Super 77 is less aggressive compared to Super 90 but could be prone to melting the Styrofoam™ the same as a fast curing resin, so I opted for low melt hot glue. 

This material melts in the gun and disperses as a bead giving enough time to press each board together and liquid enough to bond to the open cell foam. A quick and strong alternative to contact adhesive or resin.

Inset lamp to clay box top.

With all the pieces of foam trimmed to size the first thing to do was to inset the lamp into the lid of the box. A couple of diagonal lines were drawn to quickly find the center and a rough circle made, next the excess material was removed with a hacksaw blade. The lamp has a lip of approximately 1/2"  and I wanted to bevel the edge of the foam so that it would fit snug to the lamp so that I could hot glue the lamp in place and also form a seal to prevent heat from escaping too quickly. I was pleasantly surprised  on how strong the bond was and pleased that the glue had no effect on the Styrofoam™. I could build up the thickness of hot glue in layers to fill any gaps making a strong and permanent bond. 

With the most difficult item glued in place I could systematically glue the sides and the back in place and finally glue in the base. This would leave just the front face open. I had decided to cut a rebate around the edge of the front panel so that the center would inset into the opening giving a better seal against the sides. This would help to prevent heat loss and also stop the front from falling open when in use. I had not decided on how I would hinge the door and just opted for duct tape for now. Eventually I will frame the Styrofoam box with plywood and make a permanent hinged front but right now I wanted to see if it will work. I have seen other hot boxes very similar to this but the top was the loose item, needless to say each time you opened it the heat was let out.

Lamp detail in hot box.

With the framework complete, the inside of the hot box was next lined with heavy duty aluminum foil. This first prevented the inside getting too gummed up with clay while in use as it would be easy to remove and secondly, the primary reason was so that heat would be reflected off and not absorbed by the foam.

To fix the aluminum foil in place light duty spray mount was used. This none aggressive glue is tacky enough to hold the aluminum foil in place but easy to remove once it gets too covered with clay and none aggressive towards the foam. I made sure to overlap in all of the corners giving complete coverage over the Styrofoam™.

The front face or door that had been rebated to sit snugly into the opening also received its covering of aluminum foil so once shut the heat should be retained. Like I said previously I had only made provision for taping the door shut until there is time to make a more permanent arrangement. The important point is to make sure that it will work.

To help in heat transfer I cut the billets into smaller chunks so that the billets had more surface area for the heat to absorb into. I had decided on a regular 60 watt size lamp because the distance to the clay was so small and there was no fan to help circulate the heat. The trade off would be the amount of time it would take to heat up the clay.

Clay box in use showing 60 watt lamp and clay in tray.

Hot box in action with the foil covered door open and clay and tray inside.

To set the clay oven in motion I attached a regular timer inline so that it would automatically turn on and turn off. This would allow for the clay to be heated up prior to being used and would also indicate how much time was needed from initial start off.

With this size light bulb I found that the clay was workable after four hours and if clay was replaced each time a piece was removed there could be a constant supply for a small job in progress. The only time there would be a lapse would be during the initial priming of the model buck but I concluded that the interim time could be used for making templates.

I had thought about using a heat lamp or a higher wattage bulb to speed up the process but I would probably face burning the clay or drying it out very quickly making the clay unusable. For the size of project I would be making, the heating time for the clay is not a problem and finding that it works quite satisfactory for minimum cost is paramount when your budget is tight.

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