# Can Ferrite be Cleaned in an Oven?

#### jpanhalt

Joined Jan 18, 2008
7,891
I have a large plate of ferrite from a loop antenna dating from 1993. It was embedded in a foam that cannot be dissolved, but I removed most of the potting physically and then used an "all" paints remover to soften it. Then, I scraped off more of the potting foam. With enough effort and solvents, I am sure I can get it clean. I am looking for an easier way and would like to reuse the ferrite in the same style antenna at 500 kHz.

Far easier would be to cook it at 500°F (about 260°C) to depolymerize the potting. Will its properties as an antenna be changed by such heating? What if it were put through the cleaning cycle for the oven (I guess about 425°C)? I found a link that said ferrite magnets could be used to 300°C, but it didn't say what happened at higher temperatures and whether the changes were permanent.

John

#### Bernard

Joined Aug 7, 2008
5,261
I believe they were fabricated at a high temperature so do not see why they could not be reheated to around 500 deg.F. Can you sacrifice a few for a trial ?

#### jpanhalt

Joined Jan 18, 2008
7,891
I can sacrifice one, but not a few. Trouble is, if I destroy this ferrite's properties, I won't know it.

Basically I have taken an old ADF antenna and am reverse engineering it for 500 kHz for a lightning detector. I wanted the exact pinouts and internal design. Now that I have that, I would like to build my own with the same ferrite (damn stuff is expensive). Of course, I could buy another ADF antenna, but that is no fun, and the result will not be pretty as what I will make.

John

#### Kermit2

Joined Feb 5, 2010
4,158
Try using old fashion DOT3 brake fluid on the remainder of the potting. Test on a small section and let it set over night.
The potting should turn to jelly and be easily scraped off.

#### OBW0549

Joined Mar 2, 2015
2,984
Far easier would be to cook it at 500°F (about 260°C) to depolymerize the potting. Will its properties as an antenna be changed by such heating? What if it were put through the cleaning cycle for the oven (I guess about 425°C)? I found a link that said ferrite magnets could be used to 300°C, but it didn't say what happened at higher temperatures and whether the changes were permanent.
I think the limitation for ferrite magnets probably related to their Curie temperature, above which they would lose their magnetism; once lost, it isn't regained when the temperature again drops.

I'm not a materials expert, but for unmagnetized ferrite I can't see where cooking them to get the gunk off would do any harm, unless you heat or cool them too fast and they shatter.

#### jpanhalt

Joined Jan 18, 2008
7,891
Thanks for the confirmation. I have a fair bit of experience doing scientific glass blowing, and for the few times I have worked with ferrite, I was impressed with how similar it was for machining and cutting. My oven takes an hour or two to get to temperature and at least 2 hours to cool. I don't think thermal shock will be an issue.

I was worried about its RF properties, which still are a mystery to me, but presumably are due more to composition than to post manufacturing heat treatment. Although, I have read that post manufacture heating (annealing?) improves the properties, but actual temperatures used for that were not given.

John

#### Alec_t

Joined Sep 17, 2013
10,361
According to Wikipedia the Curie temp of strontium ferrite is 723K (450C), if that's of help?

#### DickCappels

Joined Aug 21, 2008
5,879
This is not a magnet, so the Curie temperature is not an issue. Thermal shock appears to not be an issue. Most likely the plate will be fine.

#### jpanhalt

Joined Jan 18, 2008
7,891
I gave it another treatment with special chemicals. Looks much better. Will give it a cleaning cycle tonight or tomorrow.

John

#### ian field

Joined Oct 27, 2012
6,543
I believe they were fabricated at a high temperature so do not see why they could not be reheated to around 500 deg.F. Can you sacrifice a few for a trial ?
AFAIK: Most (ferrite rods are extruded from a stiff paste) ferrites are sintered, that means they start off as powder that is stamped into a shape in a big hydraulic press, then they're "fired" - I don't know the temperature, but the firm I worked for pumped nitrogen into the conveyor kiln so the iron particles didn't oxidise more than specified.

If ferrites become magnetised; You can remedy that by raising them above the Curie point temperature.

A development engineer at the firm I worked for, told me they fired individual prototype samples in the microwave - although a ferrite slab would probably be at risk of fracture!

#### jpanhalt

Joined Jan 18, 2008
7,891
AFAIK: Most (ferrite rods are extruded from a stiff paste) ferrites are sintered, that means they start off as powder that is stamped into a shape in a big hydraulic press, then they're "fired" - I don't know the temperature, but the firm I worked for pumped nitrogen into the conveyor kiln so the iron particles didn't oxidise more than specified.

If ferrites become magnetised; You can remedy that by raising them above the Curie point temperature.

A development engineer at the firm I worked for, told me they fired individual prototype samples in the microwave - although a ferrite slab would probably be at risk of fracture!
Re: underlined. Interesting. I read that ferrite ages and often gets better with age. Iron will oxidize without heat. Anyway, it will be tested.

I learned what I wanted about the antenna's construction. Worst case, I will spend another $20 for an intact antenna. eBay listings are a laugh. Some people are listing used ADF stuff like it was the 1960's and the newest technology. John #### ian field Joined Oct 27, 2012 6,543 Re: underlined. Interesting. I read that ferrite ages and often gets better with age. Iron will oxidize without heat. Anyway, it will be tested. I learned what I wanted about the antenna's construction. Worst case, I will spend another$20 for an intact antenna. eBay listings are a laugh. Some people are listing used ADF stuff like it was the 1960's and the newest technology.

John
Speaking of sintered ceramics - you can make spark plug insulators glow red in the microwave.

Ferrites absorb more energy faster though.

#### jpanhalt

Joined Jan 18, 2008
7,891
The ferrite cleaned fine in the oven, but as soon as the oven started to warm -- it was not even at a medium baking temperature -- the plate cracked. The two parts popped apart by about 1 cm. The plate was apparently cold machined for a 9-pin round plug, and the break line was right were one might expect there to be stresses.

John

#### DickCappels

Joined Aug 21, 2008
5,879
Bad luck. So much for the idea that thermal shock won't be an issue

If you can get the two pieces to fit snugly together snugly (don't put glue or expoxy in the gap) it should work very much like it would if not cracked. I have done that with ferrite transformer cores with no problem.

#### Alec_t

Joined Sep 17, 2013
10,361
If this is for an antenna, does it matter if the ferrite is cracked, or even if there is a glue-thickness gap between the pieces? I wouldn't have thought a small gap would make much difference to the total magnetic reluctance, if the antenna winding can be arranged so that the two pieces are effectively in parallel rather than in series as far as the magnetic circuit is concerned.

#### jpanhalt

Joined Jan 18, 2008
7,891
That's right, it probably doesn't matter much for a ferrite loop antenna. There is quite a bit on the DX sites about "ferrite sleeve" antennas that use multiple ferrite rods -- often the rods don't even touch . Here is one study: http://www.am-dx.com/antennas/FSL Antenna Design Optimization.htm

I was still surprised that it popped at such a low temperature. The antenna was more than 20 years old. You see early breaking like that when firing ceramics and when annealing scientific glassware. On the bright side, it did come out clean and the smell in the house will be gone in a few days.

John

#### atferrari

Joined Jan 6, 2004
3,452
Hola John,

Out of curiosity, why did you try to clean it? To ensure a final size or just to have it neat and tidy?

What is the shape/layout around that plate? Bigger or smaller than the original? Rusty little knowledge about antennas here.