I have been told that an oven may be kashered without the use of a torch for Libum even if it has not had 24 hrs at rest! Is this possible and if so how?
You might be thinking of a self cleaning oven. The self cleaning cycle is sufficient to kasher it. Non self cleaning ovens may be kashered by running at the highest setting for a set period of time.
If your oven is of the self-cleaning variety, koshering it is fairly simple. All you need to do is self-clean it. If you need to kosher the oven racks too, just leave them in the oven while it self-cleans.
In a conventional oven, whether gas or electric, an oven cleaner may be necessary to remove baked-on grease. Be sure to check hidden areas, including corners, door edges, the area behind the flame burners, and the grooves of the rack shelves. If a caustic type of oven cleaner (such as Easy-Off) was used to clean the oven, and some stubborn spots remained after a second application with similar results, the remaining spots may be disregarded. Once the oven and racks have been cleaned, they may be kashered by libbun kal. The requirement of libbun kal is satisfied by turning the oven to broil or the highest setting for forty minutes. In a gas oven, the broil setting will allow the flame to burn continuously. In a conventional electric oven, the highest setting (550°F) kashers the oven. Only libbun kal is required for the oven racks since it is usual to cook food in a pan and not directly on the racks themselves.
In a continuous cleaning oven, one cannot assume that such an oven is clean because the manufacturer claims it to be continuously clean. A visual inspection is required. Since caustic or abrasive oven cleaners, e.g. Easy Off, cannot be used without destroying the continuous clean properties of the oven, a non-abrasive and non-caustic cleaner must be used to clean the oven. Grease spots will usually disappear if the top layer of grease is cleaned with Fantastik and a nylon brush. Then the oven should be turned on to 450°F for an hour, so that the continuous clean mechanism can work. If the spots do not disappear, the oven should be left on for a few hours to allow the continuous clean mechanism to deep clean, or else the spots should be removed with oven cleaner or steel wool. If the spots are dark and crumble when scratched they can be disregarded. In all of the above cases, the oven should then be kashered by turning it to the 550°F setting for forty minutes.
In a self-cleaning oven; before using the self clean cycle, one should clean the inside face of the oven door, as well as the opposing outer rim of the oven outside the gasket, since these areas are not necessarily cleaned during the cycle. Easy Off manufactures one product that is safe for self-cleaning ovens. One should ensure the gasket itself is clean on the area outside the oven seal. (Note: The gasket is sensitive to abrasion). The self-cleaning cycle will then clean and kasher the oven simultaneously. Caution: There is a potential risk of fire during the self-cleaning process, especially if there is a great collection on the bottom of the oven. It is recommended to apply Easy Off that is made especially for self-clean ovens to lift and remove the grease. The oven should not be left unattended while in the self-cleaning mode. Due to reports of fires, one should not leave other operating heating appliances i.e. dryers, toasters, coffee makers, toaster ovens, microwave ovens, or dishwashers unattended. In fact, toasters and toaster ovens should be unplugged (by pulling on the plug and not the cord) when not in use.
Most oven manufacturers instruct the consumer to remove the oven racks before self cleaning, so they will not discolor. However, one may self clean the oven with the racks inside, even though they might discolor. After the self clean cycle use a little oil on the side of the racks to easily slide them in and out of the oven. Also, there are some manufacturers that require removal of the racks before the self clean cycle can start. In this case, one should take the racks out, clean them very well, and then put them in the oven at the highest temperature (usually 550°F) for 45 minutes. This procedure kashers them.
Some ovens come with a convection feature. This feature allows for more uniform heat distribution by using a fan to circulate the heat. If the convection oven has the self-cleaning feature, it will be sufficient to kasher the fan as well. If there is no self-cleaning feature, the entire oven including the fan, while it is circulating must be sprayed with a caustic cleaner and cleaned well. The oven should then be kashered by turning it on to the 550°F setting for forty minutes.
LIBUN > Remove any visible food. Complete self-cleaning cycle with racks in place.
NON SELF-CLEANING OVEN > LIBUN
Clean all surfaces (walls, floor, doors and racks) thoroughly with a caustic cleanser (e.g., Easy Off). Pay special attention to thermostat, oven window, and edges of the oven chamber. Black discoloration that is flush with the metal need not be removed. Oven should not be used for 24 hours. Place racks in the oven and turn the oven to broil (highest heat) for 90 minutes. A broiler pan that comes in direct contact with food should not be used.Note: The method of kashering described above is based on the ruling of Rav Aharon Kotler zt’l. However, Rav Moshe Feinstein zt’l ruled that the oven must either be kashered with a blow torch, or an insert should be placed in the oven. Consult your rabbi for guidance.