Kiln Going Higher than Programmed Temp

[Earthen Hands Ceramics]

Hi ceramics community! I have a kiln question.

I inherited a Cress model e23 kiln from 2001 (automatic). It is mostly in good working condition and I have had a professional electrician configure all my wiring and test my voltage etc. and confirm the kiln is wired properly and should operate at 100%. However, I have had ongoing issues and inconsistencies with my firings. 
I use all cone 5 laguna clays and glaze - bisque to 04 slow and glaze fire cone 5 medium. 
The issue arises when I program my glaze firings - I initially select my program and things seem to be going well up until the end of the firing. The preview indicates that the kiln is trying to reach 2165 until the last ~30mins when suddenly the projected temp changes to 2181+. This temp is too hot for my glazes and has caused a multitude of issues such as blistering, over-running etc. I usually end up turning my kiln off manually at 2165 but you can imagine this is frustrating knowing I have to be very present with the kiln at the end of the firing otherwise it will not shut off automatically and over fire my work. Plus I’m unsure that if I manually shut it off that I get a controlled cooling cycle. (??)
The manufacturer help me set a custom ramp-hold program to mimic a factory cone 5 medium firing and it sometimes shuts off automatically at 2165, but sometimes does the same thing (continues to increase in temp. At the end) he has not recommended anything else or have an idea why this is happening besides possibly an issue with my voltage, which, like I said, has been ruled out by my electrician. I understand tightly packed kilns can take a bit longer sometimes, but why would my cone 5 program be climbing up to cone 6 and beyond? 

so I suppose my questions are:

why is this happening? Is this just a cress kiln thing?

does anyone have a go-to custom ramp hold recommendation for laguna cone 5 clays and glazes?

sincerely, 

madeline

(a potter who desperately  needs a kiln mentor LOL)

[neilestrick]

What type of controller does it have, and what firing schedule are you using? Specifically, what is the rate of the last ramp? If the last ramp is faster than 108F/hr, then it will need to go to a higher temp in order to achieve the heatwork of cone 5, which is why it may be raising the temp at the end.

Have you confirmed via cones that the kiln is firing hotter than cone 5?

It's also possible that your 20 year old controller is due for replacement.

[Bill Kielb]

19 hours ago, Earthen Hands Ceramics said:

does anyone have a go-to custom ramp hold recommendation for laguna cone 5 clays and glazes?

Regardless of ramps (forget about holds) if your last ramp is 108 degrees per hour for the last 200 degrees of your firing you should be able to pretty much nail your cone from the center column of the Orton chart. Having said that, if I manually  program a ramp and ending temperature, the ending temperature simply cannot change to a new number. So as Neil said above, what is the last segment speed? What controller is in use, Cress has traditionally used the Bartlett control.

It would be odd for a control to change temperature to a higher number unless it mysteriously started heating faster than planned  so it prompts the question what exactly is good voltage and amperage at full load? 

A couple other thoughts crossed my mind. Do you use the automatic cone fire program? Have you used a ramp hold and what was it? Is this a three zone kiln with three thermocouples?

2181 is an odd number (Conee 5-1/2 ish) and indicates the kiln is going something like 150 degrees per hour in the last segment so I am just curious how you have programmed your slow cool into it. Pleas post the schedules you have used, it may be helpful.